Mention mass build cancellation command in tools dir also.

Bug: 1112539
Change-Id: I4cd0774d2728b14559d7f45cab221b0c809713d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425103Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Auto-Submit: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809540}

tools/infra/README.md

+Directory of scripts needed for troopering.
+
+#### Mass cancelling builds and tasks
+
+To cancel many builds at once use the following command:
+
+```
+# Cancel scheduled builds in bucket "bucket".
+bb ls -id -status scheduled chromium/bucket | bb cancel -reason unnecessary
+
+# Cancel started builds for CI builder "builder".
+bb ls -id -status started chromium/ci/builder | bb cancel -reason "bad builds"
+```