Add a link

csharp/keys/README.md

@@ -5,5 +5,5 @@ Contents
   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.
+  As per [Microsoft guidance](https://msdn.microsoft.com/en-us/library/wd40t7ad(v=vs.110).aspx)
+  signing key should be checked into the repository.