Policy: only allow libre media

[throwaway1037]

A policy should be instated which only allows libre media.

This would allow systems like Parabola which only allow libre media to keep SuperTuxKart out of thier warning list for programs which might download proprietary sofwtare or media.

This could be implemented via SPDX license headers, whereby only addons licensed under a license which is accepted by both the FSF and the OSI will be accepted onto the official addon repository.

Also, source files must be required on all addons, to allow practical freedom.