Add LICENSE file

[dkaszews]

I had trouble finding out what is the license of this project, it is only mentioned at the very bottom of the readme. It is encouraged to add a LICENSE file in the root of the repository, where it can easily be seen and summarized by GitHub. Plus, "same license as vim" is ambiguous, as it doesn't specify which version of vim you should check, and whether it uses precisely vim license, or GPL it is based on, if latter then GPL2, GPL3 or any?

[codeinabox]

@janko could you confirm the licence used?