Document overriding defaultBuild using GitHub milestone

The docs at https://lwc-gus-bot.herokuapp.com mention:

> `defaultBuild` The build number the application is going to use by default when it creates a new work item. Notice this value might need to be changed every release.

The wording's use of "by default" suggested to me that it was possible to override the build, but the docs don't mention how.

From reading the source I see that one can use the GitHub milestone to do that:
https://github.com/forcedotcom/git2gus/blob/dbe8c53c7bf703d7607ca595ef0a473a843eb3b9/api/actions/createWorkItem.js#L73

It would be good to document this, particularly since if I'm reading the code correctly, Git2Gus always uses the milestone (if set) with no way to opt out - and will give errors if such a build does not exist in GUS. ie: Projects need to either need to use appropriately named milestones in GitHub, or not use milestones at all, otherwise Git2Gus will error.