Add a contributing page. Refs #123

Here's a first swing at a contributing page in the documentation. I'm not a Python developer and hearing about `python setup.py develop` pretty much saved me. Other potential contributors might not know the PR flow. I figured it would be cool to lay out all the steps involved to making a Fig contribution as to lower the bar of entry. 

I might still be overlooking steps still, but/so let's add them.