How do I use Sub-Repositories?
Right now MacHg does not have support for automatic dependent "sub-repos".
However you can easily store one repository inside another and then administer both separately. (In fact one might argue that this is the way things should work in any case.)
Ie it is perfectly fine to structure things in the finder so you have one repository inside another in terms of the folder structure. To manage these repositories in MacHg simply drag one of the repositories (the overarching one or the sub one) to the sidebar of MacHg and then drag the other to the sidebar of MacHg. Thus MacHg is treating the repositories as independent. Then for whichever repository you want to interact with simply click on it in the sidebar. Then as normal commit files, diff files, inspect history, etc. In this sense you are managing the repositories independently, even though one repository is located inside another in terms of the folder structure.
They are independent in the sense that for instance committing all files in the overarching repository will not commit any files in the sub-repo.
(Ideally there would be some visual indication that the repositories are linked, and if you are dealing with the parent repository it should show some visual indication of the status of the files of the child so you remember to do a commit in the child repository. This will come in time.)
You still need to do a commit from the command line of the parent when you want to lock in the fact that the parent version is using the given versions of the child sub-repositories. (Again this support will come in time...)