Add LICENSE.txt, containing Mozilla Public License v 1.1

Contents downloaded from
https://www.mozilla.org/media/MPL/1.1/index.0c5913925d40.txt, just a few
minutes ago.

I used MPL 1.1, per @SergioFierens' response to SergioFierens#22, where he said
MPL1.1 is the licence for this repo. Since then, the latest MPL is 2.0,
but we can handle updating the licence in another thread.

Fixes SergioFierens#31