If you would like to help test Flathub, you can add the repository and install apps from it. To do this, you will need to install Flatpak, if you haven't already. After that, there are two options:
If you are on Arch, Debian Stretch (or later), Endless OS, Fedora 25 (or later), Mageia 6 (or later), or OpenSUSE Tumbleweed, you can download the repository file and use GNOME Software to install it.
From the command line, run:
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
You can then browse and install applications in the normal manner. See the Flatpak command line tutorial for more information on this.
If you have an app that you would like to make available through Flathub, please see the submission guidelines.
Security or legal issues can be reported to the Flathub maintainers.