Order readme files based on how well we can get the markup right.

Summary:
Handling readmes with no extension is a bit of a hack, but seemed like a small cost.

The Big Win here is that you can commit README.remarkup and README.md and have both Phabricator and GitHub render with all the pretty markup.

Test Plan: Looked at some readme files.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10047