Add GPL-2.0 license file for Github

[saxbophone]

Project already is licensed as GPL-2.0, adding this file just makes this clear to users exploring the source via Github and Github will pick up on this, displaying a "GPL-2.0" badge on the project stats sidebar:

This is really useful to allow other developers to see what license the code is released under at a glance rather than have to hunt the information down.

Related to #78