R1:4d6aea7ce81a

Add a more complete README for GitHub

Summary: Not sure this is entirely correct or needed as a separate file, but I think GitHub's README could use some additonal information

Test Plan: Used a Markdown editor

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11542