Update the README file

analyzers/src/SonarAnalyzer.ShimLayer/README.md

@@ -2,9 +2,19 @@
 
 ## Purpose
 
-All those classes allow to use new Roslyn API while maintaining compatibility with old versions.
+Allows the usage of the new Roslyn API while maintaining compatibility with old versions.
 
-## Notes
+## Structure
 
-The code was copied from
-https://github.com/DotNetAnalyzers/StyleCopAnalyzers/tree/master/StyleCop.Analyzers/StyleCop.Analyzers/Lightup
+The project is divided into the following parts:
+- At the root level is the `Lightup` layer introduced by the stylecop analyzers. The code was copied from
+  https://github.com/DotNetAnalyzers/StyleCopAnalyzers/tree/master/StyleCop.Analyzers/StyleCop.Analyzers/Lightup excepting `Syntax.xml` and `OperationInterfaces.xml`.
+- [Syntax.xml](https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Syntax/Syntax.xml) and [OperationInterfaces.xml](https://github.com/dotnet/roslyn/blob/main/src/Compilers/Core/Portable/Operations/OperationInterfaces.xml) are copied from the Roslyn repository. They are used to generate the `Lightup` layer.
+We copy them from Roslyn and from StyleCopAnalyzers to ensure that we have the most recent version to be able to support the latest features.
+- All the additions made by SonarSource are in the `Sonar` folder.
+
+## Conventions
+
+- Keep our changes and all logic in a dedicated directory `Sonar`, using partial classes, extension methods, and external handlers.
+- Keep the namespace. Have different license headers.
+- Inject ourselves to the StyleCop code with minimal changes that are annotated with // Sonar comment everywhere.