My only suggestion is that a little example should be included in the API documentation, maybe in the Graphics.UI.Threepenny module. The examples provided in the library are nice, but sometimes you look at the API first (as I do) and it might help. Personal opinion, of course.
Also, you may point to the Graphics.UI.Threepenny.Core module in the cabal description or the Graphics.UI.Threepenny module, since I think is the best place to start reading the API docs.
EDIT: You can also point to some information about how to use FRP with threepenny-gui!
Daniel Diaz suggests: