Add some instructions for using gsutil to the buildtools/ README.
Add some instructions for using gsutil to the buildtools/ README.

R=brettw@chromium.org
BUG=430215

Review URL: https://codereview.chromium.org/740503004

azunite/chrome_buildtools,azunite/chrome_buildtools,SuYiling/chrome_buildtools,SuYiling/chrome_buildtools,SuYiling/chrome_buildtools,azunite/chrome_buildtools