Mention how to define proprietary license

[tintou]

It seems that the way forward is to use <project_license>LicenseRef-proprietary</project_license> (looking at libappstream-glib) but the documentation doesn't mention it anywhere

[hughsie]

I think @ximion wasa against this before, but I do think it would be a handy note to have in the spec.

[cassidyjames]

This is also implemented in at least GNOME Software, AppCenter, and the AppCenter fork of Pop!_Shop, plus what several apps in Flathub—plus the Flathub website itself—are using. So it seems pretty defacto across implementations.

[ximion]

@hughsie If I was against this, I don't remember it ^^ - especially since it's the way SPDX wants us to do these things and the spec already includes it in an example for driver components: https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Driver.html
So, if you have a PR for this I will happily merge it (otherwise I'll just add at the next bug squashing day for this project)

[ximion]

Duplicate of #107