This also updates the README with guidelines.

Bug: chromium:788104
Change-Id: I0ca0ea78c5990204b0242be9c7fe6368439a5dd1
Reviewed-on: https://chromium-review.googlesource.com/796311Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49713}