Add breadcrumb to docs about third-party review process.

If, in the future, someone else needs to track down how this works, this
will hopefully save them a lot of time.

Change-Id: I212ceeefaff08a69a34efe1ea808ff9061edd982
Reviewed-on: https://chromium-review.googlesource.com/c/1336489Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#608155}

docs/adding_to_third_party.md

@@ -138,7 +138,8 @@ Non-Googlers can email one of the people in
 * Add chromium-third-party@google.com as a reviewer on your change. This
   will trigger an automatic round-robin assignment of the review to an
   appropriate reviewer. This list does not receive or deliver email, so only
-  use it as a reviewer, not for other communication.
+  use it as a reviewer, not for other communication. (Internally, see
+  b/119558132 for details about how this is configured.)
 
 Please send separate emails to the eng review and security lists.