Add LICENSE file for clarity

[foxytanuki]

Hi, thanks for the great repository. The codebase and documentation are very clear, and it’s been really helpful to understand the connection between Lace wallet and midnight-node.

One small suggestion, I noticed there isn’t a LICENSE file in the repository yet. Adding one would make the usage and redistribution terms clearer for users and contributors, and could lower the barrier for adoption.

This is not urgent at all, but I think it would be a nice improvement for the project overall.
Thanks again for sharing such a solid repo!