Added licence (GPLv3)

I would highly advise putting a licence on this mod, under the GPLv3. It's already an open source project, so you might as well. The only real alternative for a project like this is the LGPL, just look them up. There are a variety of other licences, but I would advise one of those two, or the WTFPL. If you don't trust just going with the GPL, go ahead and look into it.

See a bit about the GPLv3 on this site: http://www.gnu.org/licenses/quick-guide-gplv3.html

If you don't take this as is, I would highly advise selecting a licence and putting it. Note that any licences you try to put must be open source compatible.

Don't forget to fill in the holes in LICENCE_SIGN with your information to validate the licence.