Flathub is the central place for building and hosting Flatpak builds.
To install applications that are hosted on Flathub, use the following:
flatpak remote-add flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub org.gnome.Contacts
For more information and more applications see https://flathub.org
For information on creating packages or reporting issues please see the contributing page.