Qt 6 Build Docs

[zquestz]

I don't see any documentation on how to build the Qt 6 version.

There is no Obsession.pro file, and the build steps for the Qt 4 branch don't work with the Qt 6 branch.

[zquestz]

I did figure out a way to get things built, but not sure it was optimal. The AUR now uses your Qt 6 branch!

https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=obsession-git