Adding email attribute, uncommenting deps by default, and dding some inline documentation

This PR does a few things.

1. It adds a default (empty) e-mail attribute, so people can just add the e-mail itself. It's worth noting that an empty maintainer e-mail address prevents a catkin package from building even if the maintainer tag is there.

2. It uncomments the run_depend and build_depend tags by default, since I think this is the first thing that everyone does after they run this script. Even if they plan to clean them up afterwards. It leaves the test_depend tags commented.

3. It adds a couple of lines of documentation at a high level explaining the three different dependency groups.