Its useful to be able to run clang-format on projects
outside of chromium. This change informs developers
how they can do that.

Change-Id: I01ef755aa6e5b53a0a5bb6377e92b2ffdfd17eff
Reviewed-on: https://chromium-review.googlesource.com/414204Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Sam Clegg <sbc@chromium.org>