docs: add the LLVM exceptions to our Apache 2.0 licensing

[favonia]

The LLVM exceptions, though not tested in a court yet, should provide better compatibility with GPL 2.0 and several license exceptions. In particular, from my naive reading, they should provide compatibility with the OCaml linking exception.

Thanks to the copyright assignment, the copyright owner is uniformly "The RedPRL Development Team" and we do not need approval from all individuals. However, I am still requesting reviews for social harmony.