This makes it more clear to anyone landing directly at README.md instead of
the root of the repo how to browse the source.

Change-Id: I19b7e8e2f8e55385fa0825d4268623ef318bdaa0
Reviewed-on: https://chromium-review.googlesource.com/314441Tested-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>