docs: flat-manager-client info

[eaglesemanation]

I'm new to building Flatpak apps and I wanted to use this action. For me, it was problematic to figure out what does "flat-manager token" means and where to get it. The main source of confusion was the official Flathub tutorial, because it suggests publishing manifests on GitHub and using their CI tools instead of directly publishing built app. After that, when I opened flat-manager repo I assumed that I was looking in the wrong place, because it made me think that it only hosted a server side app. I think mentioning in docs where to get flat-manager-client and how to generate token with it would make things much easier.