Git Town Tutorial

@charlierudolph @allewun 

This adds the beginning of a tutorial. I think this will be helpful to guide newcomers to get faster how Git Town is awesome. I'm not too married to the format and structure, so feel free to comment. This will also be iterated upon a few times later, so the goal here is to get something out that is good enough as a start.

Please let me know what you think!  :)
When you are done, you run `git pr`. It opens the browser to the Github page for creating a new pull request for the current branch. You update the title and description, tag the appropriate reviewers, and submit.