Add README.chromium.

So that we can add pyelftools (along with chromite) to the Chromium
checkout.

BUG=None
TEST=None

Change-Id: Icf7f161ec674bfb3a4fe964f3483794e7430797c
Reviewed-on: https://gerrit.chromium.org/gerrit/45945
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>