Add README.md to NetAnalyzers and CODEOWNERS entry (#50288)

Author: ViktorHofer

CODEOWNERS

@@ -109,3 +109,5 @@
 # dotnet-format
 /src/BuiltInTools/dotnet-format @dotnet/roslyn-ide
 /test/dotnet-format.UnitTests @dotnet/roslyn-ide
+
+/src/Microsoft.CodeAnalysis.NetAnalyzers @dotnet/dotnet-analyzers

src/Microsoft.CodeAnalysis.NetAnalyzers/README.md

@@ -0,0 +1,4 @@
+# Microsoft.CodeAnalysis.NetAnalyzers
+
+This folder contains the source code for the .NET analyzers that are part of the Microsoft.CodeAnalysis.NetAnalyzers package.
+This code formerly lived in the [dotnet/roslyn-analyzers](https://github.com/dotnet/roslyn-analyzers) repository.