Add advice to download commit-msg hook Thought this was taken care of with `gclient sync` but it seems like a manual download is needed now. Bug: angleproject:6517 Change-Id: I87499149b810c873827883628104963178a14fed Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3203171 Commit-Queue: Lingfeng Yang <lfy@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com>