README: Link to Flathub discourse
README: Link to Flathub discourse

We don't have a separate discourse for Flatpak; it seems fine to link to
the Flathub one.

Fixes https://github.com/flatpak/flatpak/issues/3602

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak