A fairly straight forward way would be to create a build comment. You can do that with the cmtool like this:
\> cmtool login userName password
\> cmtool createBuildComment 1044 "This is our gold build for release 7.0"
This will log you in to the cluster manager running on the local machine (you can use --server to contact a remote machine), and attaches a new comment to build id 1044. Note that you can attach comments to other entities as well such as build classes, agents as well as the over all server).
You can find more information about cmtool in the cmtool documentation.