Create reuse workflow for checking licenses

[ajtribick]

This is going to fail until we annotate the files accordingly, but we should at least track this.

https://reuse.software/tutorial/

Relates to #138

[375gnu]

can we avoid file duplication? i mean LICENSES/GPL-2.0-or-later.txt, which is just a copy of COPYING.

[ajtribick]

Taking a look at the Compliant projects page, I see that libexif just duplicates the main license in the COPYING file in the root directory.

Alternatively we could do what slint does - keep the actual licenses in the LICENSES folder and then use something like https://github.com/slint-ui/slint/blob/master/LICENSE.md

Windows probably would have issues if we made COPYING a symlink...

[ajtribick]

Went with the Slint approach, have added some license information