Add a licence file

@xylar and I are looking to add `cprnc` to the next version of `e3sm_unified` [[1]](https://github.com/E3SM-Project/e3sm-unified/discussions/143#discussioncomment-15078248). We've gone back and forth about how to package it, but it seems like the easiest option will be building `cprnc` on conda-forge.

Xylar is taking care of adding the [feedstock](https://github.com/conda-forge/staged-recipes/pull/31738) for `cprnc`, but one of the requirements for building on conda-forge is a license file. He's temporarily used the [license file](https://github.com/ESMCI/cime/blob/master/LICENSE.TXT) from CIME, since `cprnc` was moved to its own repo from there.

While that will work for now, we'd like to get the right license file in there long term. What would be the appropriate license file to add here?