Add LICENCE file

[triblondon]

This repo is missing a licence declaration. I'd like to reuse some of the polyfills in the polyfill service but won't be able to unless you confirm that the code is licensed under terms that permit reuse.

This file also links to the licence, which 404s:
https://github.com/webcomponents/webcomponentsjs/blob/master/CONTRIBUTING.md

[addyosmani]

Thanks for catching this. We currently include LICENSE information as part of a banner https://github.com/webcomponents/webcomponentsjs/blob/master/banner.txt prepended to files as necessary. We could improve on this with a separate LICENSE file in the repo - @jmesserly thoughts?