Suggestion: add example for a flatpak package .yml in the README for how to use the ubuntu touch runtime as a base for packaging an app

My apologies if this already is in the README and I simply missed it, but I wanted to suggest you add example for a flatpak package .yml in the README for how to use the ubuntu touch runtime as a base for packaging an app. This would make it easier to start trying to package an ubuntu touch application for flathub, which I'm trying to do right now. I need a base SDK with the lomiri dev and Qt dev and Qt web engine dev packages (Qt5) to get started, and it seems like the ubuntu touch flatpak runtime may fit the bill, but I'm not quite sure how to actually reference it in a package to test it out.

Sorry again if this is a really obvious task already covered in the documentation and I just missed it.