Mention tup's (planned) availability as a Flatpak SDK extension

[nmlgc]

I've packaged Tup for easier use during the build process of Flatpak packages, and have submitted the resulting package to Flathub:

https://github.com/flathub/flathub/pull/6000

This PR should only be merged if that one goes through, obviously.

The front page may or may not be the right place to point out that this integration exists, but this PR is primarily meant to fulfill Flathub's "contact the upstream developer" submission criterion. 🙂