message: doc: add mailing list address to CONTRIBUTING
diff: # Contributing
  When contributing to this repository, please first discuss the change you wish to make via issue,
  -email, or any other method with the community before making a change.
  +email, or any other method with the community before making a change. The developer mailing list
  +([English] | [Chinese]) should be used to discuss complicated or controversial changes before working
  +on a patch set.
  +
  +[english]: https://groups.google.com/a/nervos.org/d/forum/ckb-dev
  +[chinese]: https://groups.google.com/a/nervos.org/d/forum/ckb-dev-zh
  Please note we have a code of conduct, please follow it in all your interactions with the project.
  