R1:9bfb28253e9d

Add a phabricator code layout doc

Summary: tried to cover the basics and sprinkle in lots of class references, etc. would really appreciate feedback...! :D

Test Plan: read the docs!

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T359

Differential Revision: https://secure.phabricator.com/D2223