Add outline for complex git features to discuss in future presentations.

[jsoules]

Purpose

We've received requests to address some of the more complex features of git which are outside the scope of the current presentation. In order to track them in an organized fashion, I'm introducing a document (to be further developed later) which will outline the important features we've yet to cover.

Changed files

The PR adds a single markdown file, complex-git.md. No modifications are made to existing files, so there should be minimal impact to existing functionality.

• complex-git.md:
  • Add reference to branching
  • Add reference to tags
  • Question: Should we also include discussion of HEAD/the idea of the current state?
  • Question: Is this document fit to merge without explaining the idea of diff and patch files?

@kelle