Add license

[kpu]

We need a license. The license should generally be open, but include a clause that prevents Mozilla from using this software unless they:

1. Clearly acknowledge the source of the translation systems and software used to train them in all PR, release notes, and when users experience the system. In the initial announcement.
2. Point users looking for language support etc to a support forum run by us, where they won't be able to delete our posts.