Added .gitignore file

This file will need to be updated if we are going to be using Unity,
because the file types to ignore will definitely be different. Also, it
needs to have the game name actually inserted into the code.
-Travis
Added CONTRIBUTING.md

Documentation for how to use common Git commands and use the Github for
Windows app. This document is always a work-in-progress so it will be
updated as needed.
-Travis