Clarify license and add license file

[ekpyron]

I'd consider using your implementation in a project, but I'm a bit hesitant, since the licensing is not entirely clear. https://github.com/Benzinga/lz4js/blob/master/package.json lists ISC as license - is that accurate? Would it be possible to add a COPYING or LICENSE file to the repository to clarify this beyond any doubt :-)? I'd very much appreciate it :-).