version-control/instructor.md: Add a 'Tool choices' section

Software Carpentry is about how to develop software, which is largely
independent of the tooling for a particular task.  This branch only
contains the generic motivation and introduction for version control.
That's nice, but a particular Software Carpentry course or boot camp
*is* going to want to teach a specific tool, and we want instructors
to easily be able to find and integrate notes for the tool they
choose.

This setup doesn't assume that there is a single canonical set of
notes for a given tool.  If you don't like the branches I've listed,
write up your own.  If you tell me about it, I'll add a link to your
branch as well, and we can try and work up some short descriptions to
help new instructors differentiate between the two approaches.