Add reference to git magic

Possibly might be useful to add reference to something like [git magic](http://www-cs-students.stanford.edu/~blynn/gitmagic/) it has useful things like chapter 8 & appendix A that are not so common to be gathered in the same space

_Originally posted by @sudodoki in https://github.com/kottans/git-course/issues/1#issuecomment-527112033_

Open the linked Git Magic resource and review chapter 8 and Appendix A, then locate the course's existing reference or resource list. Add the Git Magic reference in the appropriate place and verify that those sections are clearly included for learners.

Written by the indexing model from the issue text.