Update the translations document

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