Add a license file

[kink-kat]

Currently the only place the licensing for the project is mentioned (as far as I'm aware) is in the package.json file, which lists that the project is licensed under the MIT license. In my opinion, this is a bit out of the way, and I was originally going to file a issue to add a license before I wound up finding it there.

To make the licensing for this project more visible, I recommend creating a LICENSE file in the root directory of the project.