Add a note to //testing/buildbot/OWNERS regarding CQ changes.

Change-Id: Ib5b4cfc157715273115baf6907fbf8211662bbe2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159747Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761101}

testing/buildbot/OWNERS

 # This is reserved for people that are familiar with how Chromium builders
 # run tests and understand the implications of changing these files.
 
+# If you're making changes that affect CQ builders (like adding new tests or
+# compile targets) please get a review from someone in //infra/OWNERS.
 file://infra/OWNERS
 
 bsheedy@chromium.org