Add a document to explain how to add a feature flag (chrome://flags)

TBR=asvitkine@chromium.org

Bug: N/A
Change-Id: I53bda41c047ed970417df5a5ca308855cbccbde5
Reviewed-on: https://chromium-review.googlesource.com/594754
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492501}
2 files changed