Add link in cipd_manifest.txt to help people figure out which commit to
use.

R=jchinlee@chromium.org

Bug: 808836
Change-Id: I2c03c6a25e79363792a85d976d2a245b0ba6b5b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1633173Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>