Add a licence file

[andrewdnolan]

@xylar and I are looking to add cprnc to the next version of e3sm_unified [1]. 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 for cprnc, but one of the requirements for building on conda-forge is a license file. He's temporarily used the license file 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?

[xylar]

If you all would like, I can make a PR to add the license file. I just didn't want to presume that was welcome.