[aur-dev] Git workflow changes

[Lukas Fleischer]

To simplify the preparation of proper maintenance releases, I just
created a separate maintenance branch ("maint"). From now on, all
maintenance-related commits will be merged into this branch.

I also decided to delete the "stable" branch which used to be nothing
more than a reference to the latest release. Follow the "maint" branch
or checkout the latest annotated tag instead.