R1:ae97617e3621

Reformat README as Remarkup

Summary: We have Remarkup, so why not use it?

Test Plan: See D9946.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9948