Diffusion - add ToC for readme files

Summary: see title. Fixes T4549.

Test Plan: made a readme that had some headers and observed a nice ToC

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: aran, epriestley, Korvin

Maniphest Tasks: T4549

Differential Revision: https://secure.phabricator.com/D8490