Add GPLv3 license in markdown

Closed username-removed-25949 requested to merge (removed):license-file into master

This MR adds the GPLv3 as a license file.

Just because it seemed weird to only mention it at the bottom of the readme and because we already have a link to it on the project page.

The License file has been copied from https://github.com/TheFox/GPLv3.md

/cc @ayufan