Archive corefx repository

CONTRIBUTING.md

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+THIS REPO IS ARCHIVED AND DOES NOT ACCEPT PULL REQUESTS.
+
+Active development has moved to https://github.com/dotnet/runtime. We welcome
+ you to visit us there. For details, please see [README.md].

README.md

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+Going forward, the .NET team is using https://github.com/dotnet/runtime to
+develop the code and issues formerly in this repository.
+
+Please see the following for more context:
+
+[dotnet/announcements#119 "Consolidating .NET GitHub repos"](https://github.com/dotnet/announcements/issues/119)