Update the documentation on ANGLE's two reviewer policy. BUG=angleproject:3704 Change-Id: I3df6f7935397b4a98db0eed569868def3e0fd701 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1707113 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>