doc: convert README to markdown and update CONTRIBUTING.md

Fixes #9233

Change-Id: If64480cbfaafe44025fcdcc0f20d76c995ffb3a1
Reviewed-on: https://go-review.googlesource.com/1296
Reviewed-by: Russ Cox <rsc@golang.org>
doc: update README.md and CONTRIBUTING.md in response to review comments

I didn't 'add' these files before my final mail.

Change-Id: Id6f25629638e58421a3dee7632aec38aad9b1970
Reviewed-on: https://go-review.googlesource.com/1340
Reviewed-by: Andrew Gerrand <adg@golang.org>