Add license file

[twils0]

I'm a big fan of react-pivot, but I would like to make it more of a controlled component and allow for more style customization. I would have gone the route of submitting several pull requests, but I would like to use the Airbnb style guide, ES6, and webpack.

With this divergence in mind, I thought I would just create a new repo: "react-pivot-plus". I noticed you do not have a license file, though your readme lists "MIT" as your license. I would like to copy the wording of your license to properly attribute my new project to you. Of course, I will mention my project is a fork of react-pivot in several places throughout my repo.