Add documenation on the alert list JSON format.
Add documenation on the alert list JSON format.

BUG=catapult:#3222
TBR=simonhatch@chromium.org

Review-Url: https://codereview.chromium.org/2893603002

catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult-csm