Add note about microsoft archive repo in readme (#16378)

* Add note about microsoft archive repo in readme

* Update README.md

* Apply suggestions from code review

Co-Authored-By: Maira Wenzel <mairaw@microsoft.com>

* Fix lint error

Co-authored-by: Maira Wenzel <mairaw@microsoft.com>

README.md

@@ -12,9 +12,7 @@ We welcome contributions to help us improve and complete the .NET docs. This is
 
 To contribute, see the [Projects for .NET Community Contributors](https://github.com/dotnet/docs/projects/35) for ideas. The [Contributing Guide](CONTRIBUTING.md) has instructions on procedures we use. 
 
-If you're interested in helping migrate existing code that targets the .NET Framework from [Code Gallery](https://code.msdn.microsoft.com) to .NET Core applications stored in our [samples repository](https://github.com/dotnet/samples) and downloadable from the [Samples Browser](https://docs.microsoft.com/samples/browse), see the [Code Gallery migration](https://github.com/dotnet/docs/projects/88) project. 
-
-We anticipate that [Xamarin](https://docs.microsoft.com/xamarin), [Mono](http://docs.go-mono.com/?link=root%3a%2fclasslib) and [Unity](https://docs.unity3d.com/Manual/index.html) will also use this documentation.
+If you're interested in helping migrate existing code that targets the .NET Framework from the [retired Code Gallery](https://docs.microsoft.com/teamblog/msdn-code-gallery-retired) site to .NET Core applications stored in our [samples repository](https://github.com/dotnet/samples) and downloadable from the [Samples Browser](https://docs.microsoft.com/samples/browse), see the [Code Gallery migration](https://github.com/dotnet/docs/projects/88) project. The code gallery samples were moved to the [Microsoft Archive](https://github.com/microsoftarchive?q=msdn-code-gallery) organization.
 
 This project has adopted the code of conduct defined by the Contributor Covenant
 to clarify expected behavior in our community.