Add LICENSE
Just to be on the safe side, we should add a license to this repo. Ordinarily we slap BSD 3-Clause Revised into everything involving ELSI, but that license doesn't apply here since this isn't a software package.
I'm currently thinking that we should do a Creative Commons license, either CC BY 4.0 or CC0 1.0. I'm leaning towards the former, since computational materials researchers should be citing the sources for their geometries anyway.