Add branch date guidance to documentation.

This is a set of guidelines that the Blink/Web Platform team leads
came up with, based on experience and input from TPMs.

R=amineer@chromium.org
BUG=

Change-Id: Iff53ce441ab46bd52ada5285ad94af57dce27b18
Reviewed-on: https://chromium-review.googlesource.com/693345
Reviewed-by: Alex Mineer <amineer@chromium.org>
Commit-Queue: Dimitri Glazkov <dglazkov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511203}
3 files changed