Add instructions and a quick reference entry for checking whether a
DevTools commit has rolled into chromium/src and which release version
or channel it is deployed to by querying the Chromium Dash API.

Bug: None
Change-Id: Ie9a033ea88a06b92a77cd71da61a5ec3d4b6a638
TAG=agy
CONV=492c6546-ae51-47d5-bace-bb2c519dc842
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/8069238
Auto-Submit: Yang Guo <yangguo@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>