Gonna add a small warning in chromium's recipe when this footer is
used. Add it here so we can refer to the footer as a constant in
tryserver recipe_mod.

Bug: None
Change-Id: Ie4f9d1d208c0dad8d0df8f07b74e761da1051a1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2864005Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>