Add LICENSE file

[jph00]

Thank you for this great work! I wonder if you'd be able to add a LICENSE file to this repo? Currently, the setup.py file says that it's under the MIT license, which is the license used for the original lucidrains repo, but most of the other files don't have any license specified. Adding a LICENSE file would make it clear under what terms people are allowed to use your code, which would be really helpful!