Add LICENSE file

[nicholas-rh]

Thanks for making your library open source. I'm looking to package it for Fedora, and its recommended that packages include a separate file which has the license text. The project license is pretty clear, but it would be helpful for documentation purposes to include this in the root of the repository.

[nicholas-rh]

Here is the package submission request https://bugzilla.redhat.com/show_bug.cgi?id=2217970