Add documentation about suppressing test failures Bug: angleproject:6462 Change-Id: Ia46f58ea66ec3e2b50fca91a645fe0e8969689d0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3192171 Commit-Queue: Ian Elliott <ianelliott@google.com> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>