Update LICENSE

Per discussion on Mattermost, we are going to keep this documentation public on the web at http://opencraft.com/doc/handbook/ and http://opencraft.com/doc/ but make the repository and Merge Request discussions private.

At the same time, it makes sense to change the license to one more suited to documentation and which does not require the "source code" to be provided. Though we'll gladly provide a copy of the source code to anyone who asks.