[Doc] Add documentation for contributing users
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing
Changes follow the draft in https://docs.google.com/document/d/1ka1rBASGXDJ1ibU7O-5S6ue_na0EXZicYi4GZCtYnA0/edit?usp=sharing