Add adding submodules

[NicMcPhee]

The new bats-core implementation of Bats is heavily dependency on git sub-modules for its libraries. "Forking" repos using templates (which is the default for GitHub Classroom) mangles/loses some sub-module information, though, so something will have to be changed to make the labs that use Bats work with bats-core.

One option is to use the "source import" option in GitHub Classroom, but that's slow and not super reliable.

Another, suggested by @floogulinc, is to use this as an opportunity to explain a little about git sub-modules and have them (re)add the modules after they make their copy via GitHub Classroom. I like this idea, and think it would make a lot of sense.

[NicMcPhee]

This is now pretty old, so it's possible that submodules are better handled when making copies from templates. We should confirm what does or doesn't happen in GitHub Classroom before we start making a bunch of changes.

[NicMcPhee]

May want to link to the bats-core tutorial, which at least mentions that the submodules are there.

If we do add some documentation on sub-modules, maybe that should be in a separate file?