Adding Flatpak to README

[superduperuser]

Description

I considered making this an issue, but I figured I could just write the change, and you can decline the pull request if you don't want it.

I only noticed today that Minigalaxy has what seems to be an official & maintained release on Flathub. However, it is not mentioned on the README/website. I added a section under Installation for Flatpak like the other entries. This would help make sure people don't miss this option.

Checklist

• [ ] CHANGELOG.md was updated (format: - Change made (thanks to github_username))

[sharkwouter]

Thanks! That's appreciated!