Add ReviewBoard notifications via a new WebHook in Rocket.Chat
In Rocket.Chat go to Administration > Workspace > Integrations and create New Integration.
Select Incoming WebHook.
Follow all instructions like Enable, give it a name, link to channel etc. Set Enable Script to true and enter the contents of this script.
Click Save changes and copy the WebHook URL (added just below the script box).
In ReviewBoard, configure a new WebHook per these instructions.