Add a basic CSS doc

Summary: Fixes T3426. This describes all the weird stuff we've got, at least. We can expand this as we get more contributors or after writing CSS lint.

Test Plan: Read document.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3426

Differential Revision: https://secure.phabricator.com/D8720