Add README docs for XML libraries

src/libraries/System.Private.Xml/README.md

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+# System.Private.Xml
+This is the primary XML (eXtensible Markup Language) assembly. It provides standards-based support for processing XML.
+
+It provides core implementations of various types including [`XMLReader`](https://learn.microsoft.com/dotnet/api/system.xml.xmlreader), [`XMLWriter`](https://learn.microsoft.com/dotnet/api/system.xml.xmlwriter), [`XMLDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xmldocument), [`XMLSerializer`](https://learn.microsoft.com/dotnet/api/system.xml.serialization.xmlserializer), and more. These types are exposed via the various `System.Xml[.*]` assemblies
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Source
+
+* XmlReader and XmlWriter: [../System.Xml.ReaderWriter](../System.Xml.ReaderWriter)
+* XmlDocument: [../System.Xml.XDocument](../System.Xml.XmlDocument)
+* XmlSerializer: [../System.Xml.XDocument](../System.Xml.XmlSerializer)
+
+Higher level APIs for performing LINQ operations on XML entities:
+* LINQ to XML: [../System.Private.Xml.Linq](../System.Private.Xml.Linq)
+* XDocument: [../System.Xml.XDocument](../System.Xml.XDocument)
+* XPath: [../System.Xml.XPath](../System.Xml.XPath), [../System.Xml.XPath.XDocument](../System.Xml.XPath.XDocument)
+
+APIs to support the creation and validation of XML digital signatures:
+System.Security.Cryptography.Xml: [../System.Security.Cryptography.Xml](../System.Security.Cryptography.Xml)
+
+## Deployment
+The XML processing libraries are included in the shared framework and also shipped as NuGet packages:
+* [System.Xml.ReaderWriter](https://www.nuget.org/packages/System.ReaderWriter)
+* [System.Xml.XmlDocument](https://www.nuget.org/packages/System.XmlDocument)
+* [System.Xml.XSerializer](https://www.nuget.org/packages/System.XSerializer)
+* [System.Xml.XDocument](https://www.nuget.org/packages/System.XDocument)
+* [System.Xml.XPath](https://www.nuget.org/packages/System.XPath)
+
+The System.Security.Cryptography.Xml library is shipped as a NuGet package as part of .NET Platform Extensions.
+* [System.Security.Cryptography.Xml](https://www.nuget.org/packages/System.Security.Cryptography.Xml/)

src/libraries/System.Xml.ReaderWriter/README.md

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+# System.Xml.ReaderWriter
+This assembly surfaces [`XmlReader`](https://learn.microsoft.com/dotnet/api/system.xml.xmlreader), [`XMLWriter`], and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.ReaderWriter` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.ReaderWriter).

src/libraries/System.Xml.XDocument/README.md

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+# System.Xml.XDocument
+This assembly surfaces [`XDocument`](https://learn.microsoft.com/dotnet/api/system.xml.linq.xdocument) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.XDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XDocument).

src/libraries/System.Xml.XPath.XDocument/README.md

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+# System.Xml.XPath.XDocument
+This assembly surfaces extensions for navigating XML documents, including the [`XDocumentExtensions`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xdocumentextensions) class. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.XPath.XDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XPath.XDocument).

src/libraries/System.Xml.XPath/README.md

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+# System.Xml.XPath
+This assembly surfaces [`XPathDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xpathdocument) and [`XPathException`](https://learn.microsoft.com/dotnet/api/system.xml.xpath.xpathexception). See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.XPath` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XPath).

src/libraries/System.Xml.XmlDocument/README.md

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+# System.Xml.XMLDocument
+This assembly surfaces [`XMLDocument`](https://learn.microsoft.com/dotnet/api/system.xml.xmldocument) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.XMLDocument` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XMLDocument).

src/libraries/System.Xml.XmlSerializer/README.md

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+# System.Xml.XMLSerializer
+This assembly surfaces [`XMLSerializer`](https://learn.microsoft.com/dotnet/api/system.xml.serialization.xmlserializer) and related types. See the [`System.Private.Xml` README file](../System.Private.Xml/README.md) for an overview of the XML-processing libraries the framework provides.
+
+Documentation can be found at https://learn.microsoft.com/dotnet/standard/serialization/introducing-xml-serialization.
+
+## Contribution Bar
+- [x] [We only consider lower-risk or high-impact fixes to maintain or improve quality](../../libraries/README.md#primary-bar)
+- [x] [We don't accept refactoring changes due to new language features](../../libraries/README.md#secondary-bars)
+
+## Deployment
+The `System.Xml.XMLSerializer` library is included in the shared framework and also shipped as a [NuGet package](https://www.nuget.org/packages/System.Xml.XMLSerializer).