Add a license file

@wlach asked on IRC if he could repurpose this bootcamp, to which I replied "of course!" ![screen shot 2014-12-19 at 16 08 52](https://cloud.githubusercontent.com/assets/90871/5511363/7786c5e8-879a-11e4-9572-bdc7d5f14e4a.png)

But I found the question odd! Turns out the license is not mentioned anywhere in the repo itself, or at least not i the top-level or README. The work was already CC-BY-4.0, so I just added a link to the license from the README and a file at the root explaining it's cool to use our work.