[Packaging] Upload app to Flatpak Hub

[Alan Zanatta]

Hi,

Linux users don't have an easy way to install the app on a distribution (either .deb, .rpm, .AppImage, snap or flatpak). I propose for Excalidraw to focus on Flatpak because it's a decentralized package manager which focuses on Desktop App, integrates well with Electron and works well cross-distribution (one package can be distributed to most linux users).

Here is the process to build a flatpak https://docs.flatpak.org/en/latest/electron.html and this is the mainstream repository https://github.com/flathub/flathub/wiki/App-Submission .

I've never built any flatpak package but it seems to be quite fast. I can help, I'm not an expert with Electron packaging and especially not with its best practices but let me know if you're interested with the proposal.

Thank you for building this awesome tool !