REUSE compliance

We'll want this sound theme to be REUSE-compliant. That means adding a .license file next to every file in the repo (following the pattern you can see in https://invent.kde.org/plasma/oxygen-sounds/-/tree/master/sounds), or else having a .reuse/dep5 file that lists the licenses of everything there (following the pattern you can see in https://invent.kde.org/plasma/plasma-welcome/-/blob/master/.reuse/dep5).