Add a CONTRIBUTING.md for the distributed package. (#44224)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44224
The purpose of this file is to help developers on PT distributed get
upto speed on the code structure and layout for PT Distributed.
ghstack-source-id: 111644842
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D23548377
fbshipit-source-id: 561d5b8e257642de172def8fdcc1311fae20690b
