Add a notice about licensing to CONTRIBUTING.md

This way we won't have to track everyone down again when we actually get around to relicensing :)
How so? If it's "GPLv2+" then you may not be able to use it under GPL3 terms if there is code it depends on that is GPL2, sure. But it still is GPL2+ and if the GPL2 deps are removed, then you could use it under GPL3.