Make it more obvious that Reference Source is read-only

Author: terrajobst

README.md

@@ -5,4 +5,6 @@ that represent a subset of the .NET Framework.  This subset contains similar fun
 developed in [.NET Core](https://github.com/dotnet/corefx).  We intend to consult the referencesource repository as we develop
 .NET Core.  It is also for the community to leverage to enable more scenarios for .NET developers. 
 
-Please note that the referencesource repository is read-only.  Questions and pull requests should be done through [.NET Core](https://github.com/dotnet/corefx).
+**Please note that the referencesource repository is read-only**. [See this blog post](http://blogs.msdn.com/b/dotnet/archive/2014/11/12/net-core-is-open-source.aspx) for the rationale.
+
+Questions, bugs, and pull requests should be done through [.NET Core](https://github.com/dotnet/corefx).