Add Lumi on Flathub

[lcruggeri]

Hello, i saw your project and i felt it's a chance it could be on flathub or available as a flatpak for use in many distros without having to compile it. But i can wait, i saw this project is really new...

[NaviTheCoderboi]

But this won't work on environments not supporting wlroots as it makes use of wlroots protocols.

I'll have it in my radar but my first priority is to get it on arch linux AUR repository.