Add Atom editor as an option

Apologies for being a tiny bit of a company shill here, but I wanted to add Atom because it's
1. free & open-source
2. hackable with HTML, CSS, and JavaScript, which might be cool for RailsBridge grads looking for challenges to apply their new skills to.
Merge branch 'master' of https://github.com/eanakashima/docs into eanakashima-master