Added GPLv3 license

[jonbarrow]

This PR adds the GPLv3 license, bringing this repo up to date with Nimble and Martini

We need permission from the following people to continue

• [x] @ashquarky (permission granted via Discord)
• [x] @Fangal-Airbag (permission granted via Discord)
• [x] @shutterbug2000 (permission granted via Discord)
• [x] @CaramelKat (permission granted via Discord)
• [x] @jonbarrow (duh I made the PR)

[jonbarrow]

All contributors have granted permission to add this license. Merging