Add firebase-value event to documentation

[maria-le]

The firebase-value event is not in the documentation as far as I can tell. So I think it needs to be added. Along with all the other available events.

[MeTaNoV]

+1

[Whyser]

Along with some documentation or example for using the query object mentioned in the documentation. Took me ages to figure out I could write this.$.myRef.query.set(...). We shouldn't have to dig through the whole javascript-documentation for basic functionality. :/