Add license

[rolandtritsch]

https://help.github.com/en/github/building-a-strong-community/adding-a-license-to-a-repository

Would suggest to use the MIT license.

[J3RN]

That's the license that I typically use 😄 However, Melpa recommends GPLv3, though any "GPL compatible license" should work. I'll do a little bit more research and choose an appropriate license. Thanks for pointing this out!