Add an example of how to make a pull request

[awfrancisco]

I feel like we should just take the entire CONTRIBUTING.md file from College Scorecard and include it here as a great example of a way to teach newbies about GitHub and welcome contributions. https://github.com/18F/college-choice/blob/dev/CONTRIBUTING.md

h/t to https://twitter.com/vtcraghead/status/643609201126834176

[melodykramer]

Would love that - do you want to add?