Add Intro to FRP post

Really interested in knowing wether this actually makes FRP easier to understand too if you don't know FRP.

And if you do know FRP please fact check me

I think revisions should work fairly well here, but we'll see.
An important note is to not make comments on the diff, but the PR otherwise when you update the pr with the changes the comment doesn't hide itself automatically (I don't think)
It wouldn't be wrong, but I'd be afraid of the notions people associate with models that might make it a bit awkward to understand.

What about calling it app state? Would that make sense?