README suggestions

Other suggestions in addition to those made in the file itself:

* In the Overview section I'd more precisely define what it means for a signal to depend on another (at a logical level).
* The analogy of a spreadsheet is helpful for learning FRP the first time.
* Maybe call out the similarities between event handlers and observers.
* It's unclear what 'external effects' are in the sentence 'Signals should not have any external effects in their definition.' Is there a more precise way to say this?

Overall, really well written and easy to understand, at least for someone who's had some basic exposure to FRP.
Merge pull request #1 from maxh/patch-1

README suggestions