What is Flatpak?
Flatpak is a universal package format for Linux desktop applications. It is available on most Linux distributions. It allows you to install and run applications in a sandboxed environment, separate from the rest of the system. This sandboxing gives you more control over the dependencies of your applications.

What is Flathub?
Flathub is a centralized repository of Flatpak apps. You can use it to install and update apps on your system.

About this PR
This PR creates a new folder on the project root named "flatpak" and creates inside the following flathub required files:

More info about it on README.md.

What happens next?

1. You need to create a new tag for the more recent release to be used by the flathub as a source for the flatpak package;
2. The app's install script needs to be modified and uploaded to the app initial subscription on the flathub;
3. After the flathub moderators approves the subscription, a fresh new home and git repo to the app will be published there (it will only have the app's install yaml script file).

As an example, see the published OpenMSX flathub home here. Pay special attention to the Statistics and Country Statistics tabs, it's a very good meter to the app's use around the world.

More info about it on README.md.