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!  :)
Implemented except open a browser **with** a page instead of **to**. 

> When you are done, you run `git pr`.
> It opens your browser with the GitHub page for creating a new pull requests.
> You fill out the title and description, tag the reviewers, and submit.