Publish on flathub

[skadge]

flathub is becoming the de-facto distribution platform for cross-distro Linux application distribution.

https://flathub.org/

It would be great for flatfab to be available there, as it would significantly ease the discovery and installation of flatfab on linux distributions.

Once PR #20 is merged, the process should be relatively straightforward, and I'm happy to make the initial submission.

Details are here: https://docs.flathub.org/docs/for-app-authors/submission
The flatpak manifest in #20 already passes the various tests.

The main step for which I'd need support from @JamesMcCrae is for ownership verification: a small file needs to be added to https://flatfab.com for flathub to confirm the project ownership:

https://docs.flathub.org/docs/for-app-authors/verification

[JamesMcCrae]

@skadge Re: verification file. No problem, feel free to send me the file via email and I'll get it added to the website.