Make licensing more clear.

You already mention the licensing in the README, however, having a LICENSE or COPYING file in the repository would make it easier to spot what license is used.

Additionally, the individual code files are currently having a header which smells like "All Rights Reserved". Ideally they'd receive a file header which would restate the license (GPL header and a note that commercial licenses are available).

Last but for sure not least, there is no hint as to how contributing works. As you're offering a commercial tier, you'll require a CLA from any contributors (otherwise you wouldn't be able to sell a non-GPL commercial tier). That should be reflected in the README (or in a CONTRIBUTE file).