Add LICENSE

Right now the project has no license, which makes it unattractive to contribute to.

Lateron you also might look into having a CLA (contributor's license agreement), so one entity can own the copyright to the project, which allows for easy switching of the license later, although I don't think this is relevant when the project is liberally licensed (i.e. MIT or Apache 2.0 license).

I suggest to use the MIT license.