Add License file

[ert485]

There is no license file in this repo (three_dart package does have a license though).

I would like to be able to develop commercial software that uses the tools in here, and I would like to be certain that I am using it in a legally safe way.

Thanks @wasabia for making these js => dart rewrites. It feels a lot better to develop with the well known three.js format, rather than creating everything from scratch like some of the other 3D flutter packages are doing.