doc: add mailing list address to CONTRIBUTING

Author: doitian

CONTRIBUTING.md

@@ -1,7 +1,12 @@
 # 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.