Merge commit '36bca61764984ff5395653cf8377ec5daa71b709' as 'libs/protobuf'

libs/protobuf/csharp/keys/README.md

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+Contents
+--------
+
+- Google.Protobuf.public.snk:
+  Public key to verify strong name of Google.Protobuf assemblies.
+- Google.Protobuf.snk:
+  Signing key to provide strong name of Google.Protobuf assemblies.
+  As per [Microsoft guidance](https://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.110).aspx)
+  signing key should be checked into the repository.