Add LICENSE

Clear up the licensing. I used the unlicense template -- we might go with something a bit more copyleft, but even by requiring a simple CC-BY the "BY" part would legally require thesis authors to acknowledge this template in their theses in case they get anyhow published, which I find completely weird. CC0 template is completely unwieldy for this purpose, and the simple unlicense was recommended by gitlab.

We may eventually migrate towards https://en.wikipedia.org/wiki/WTFPL

Closes #1 (closed).