add private key for strong name assemblies

csharp/keys/README.md

@@ -2,4 +2,8 @@ Contents
 --------
 
 - Google.Protobuf.public.snk:
-  Public key to verify strong name of Google.Protobuf assemblies.
+  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 https://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.110).aspx
+  signing key should be checked into the repository.