Add markdown Readme for System.Text.Json

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -22,6 +22,7 @@ System.Text.Json.Nodes.JsonNode
 System.Text.Json.Nodes.JsonArray
 System.Text.Json.Nodes.JsonObject
 System.Text.Json.Nodes.JsonValue</PackageDescription>
+    <PackageReadmeFile>readme.md</PackageReadmeFile>
   </PropertyGroup>
 
   <!-- DesignTimeBuild requires all the TargetFramework Derived Properties to not be present in the first property group. -->
@@ -319,6 +320,7 @@ System.Text.Json.Nodes.JsonValue</PackageDescription>
     <Compile Include="System\Text\Json\Writer\Utf8JsonWriter.WriteValues.UnsignedNumber.cs" />
     <Compile Include="System\ReflectionExtensions.cs" />
     <Compile Include="$(CommonPath)System\Obsoletions.cs" Link="Common\System\Obsoletions.cs" />
+    <None Include="readme.md" Pack="true" PackagePath="\"/>
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">

src/libraries/System.Text.Json/src/readme.md

@@ -0,0 +1,54 @@
+## About
+
+Provides high-performance and low-allocating types that serialize objects to JavaScript Object Notation (JSON) text and deserialize JSON text to objects, with UTF-8 support built-in. Also provides types to read and write JSON text encoded as UTF-8, and to create an in-memory document object model (DOM), that is read-only, for random access of the JSON elements within a structured view of the data.
+
+The `System.Text.Json` library is built-in as part of the shared framework for .NET Core 3.0+ and .NET 5+. You only need to install it as a package for earlier .NET Core versions or other target frameworks.
+
+For more information, see [JSON serialization and deserialization in .NET](https://docs.microsoft.com/dotnet/standard/serialization/system-text-json-overview).
+
+## Example
+
+The following example shows how to serialize and deserialize JSON.
+
+```
+using System;
+using System.Text.Json;
+
+class Person
+{
+    public string Name { get; set; }
+    public string Surname { get; set; }
+    public DateTime BirthDate { get; set; }        
+}
+
+class Program
+{
+    static void Main()
+    {
+        var person = new Person();
+        person.Name = "John";
+        person.Surname = "Smith";
+        person.BirthDate = new DateTime(1988, 4, 20);
+
+        // Serialize object to JSON:
+        string jsonString = JsonSerializer.Serialize(person);
+        Console.WriteLine(jsonString);
+        // Output:
+        // {"Name":"John","Surname":"Smith","BirthDate":"1988-04-20T00:00:00"}
+
+        // Deserialize object from JSON:
+        Person personDeserialized = JsonSerializer.Deserialize<Person>(jsonString);
+        Console.WriteLine($"Name: {personDeserialized.Name}");
+        Console.WriteLine($"Surname: {personDeserialized.Surname}");
+        Console.WriteLine($"BirthDate: {personDeserialized.BirthDate}");
+    }
+}
+```
+
+## Commonly Used Types
+
+- [System.Text.Json.JsonSerializer](https://docs.microsoft.com/dotnet/api/system.text.json.jsonserializer)
+- [System.Text.Json.JsonDocument](https://docs.microsoft.com/dotnet/api/system.text.json.jsondocument)
+- [System.Text.Json.JsonElement](https://docs.microsoft.com/dotnet/api/system.text.json.jsonelement)
+- [System.Text.Json.Utf8JsonWriter](https://docs.microsoft.com/dotnet/api/system.text.json.utf8jsonwriter)
+- [System.Text.Json.Utf8JsonReader](https://docs.microsoft.com/dotnet/api/system.text.json.utf8jsonreader)