Specify license

[AgostinoSturaro]

What's the license covering this project?
If nothing is specified, then it legally defaults to "all rights reserved".
Which means nothing useful can be done with it.

In case it should still be decided, what about using

• Apache 2.0 for the code
• CC-BY 4.0 for assets and doc?

It would align it with what LibGDX uses. Basically,

• do what you want with the code
• don't hold the author(s) liable if something breaks
• give credit to the author(s) as the original creators