Add a License

[Manuel Drehwald]

We should agree on a License @bytesnake.
I would prefer MIT / Apache2, as it's the default for the rust side.
However, we should verify that this isn't conflicting with LLVM's / Enzyme's Apache2-with-LLVM-exceptions licsense.

[Lorenz Schmidt]

yes I'm fine with the MIT / Apache2 duo

[Manuel Drehwald]

Closed by #5