Create README.md

README.md

+This repo was originally exported from http://code.google.com/p/protobuf-csharp-port,
+and represents the latest "legacy" version of protobuf-csharp-port.
+
+This project has now been folded into the [main Google Protocol Buffers project](http://github.com/google/protobuf),
+and is being revamped to support proto3. It is not expected to be fully backwardly-compatible with
+the code in this repo - in particular, some of the C#-specific options may not be supported in the new
+codebase, and some of the generated code may differ. A new major version offers an opportunity to spring-clean,
+as it were.
+
+This repo exists as an acknowledgement that not everyone will want to immediately use the newer codebase,
+and that they may wish to make changes to the legacy codebase. I do not expect to make any changes within this
+repo, but it can be forked for further changes.