Summary:
Fixes T8616. The rules for contributors have come up a few times recently, so update this document to give more complete advice.
Also try to do a better job with "adding new classes" (previously: libphutil libraries blah blah no one cares).
Test Plan: Read documents.
Reviewers: btrahan, joshuaspence
Reviewed By: btrahan, joshuaspence
Subscribers: joshuaspence, epriestley
Maniphest Tasks: T8616
Differential Revision: https://secure.phabricator.com/D13358
