Summary:
(from @njormrod) This changes the READMEs to be .md,
appropriate for github. I viewed the docs on github, and they looked
WAY better as .md files.
Signed-off-by: Nicholas Ormrod <njormrod@fb.com>

Test Plan: view on github

Reviewed By: markisaa@fb.com

Subscribers: fugalh, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D1999531

Tasks: 6783581

Signature: t1:1999531:1429220432:68c6d8dc70806d993c83cacce6369cb7e6e964c8