Update README.md/HACK.md to mention github mirror

Change-Id: I56d992d861a99a035c4fe57c0e40a706a4d748d8
Reviewed-on: https://chromium-review.googlesource.com/175842
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>