Added FuturesUnordered chapter

[capnhawkbill]

I added the chapter about FuturesUnordered, it's a bit short but I think it gets the message across

[tmandry]

I think this could be even more concise, something like

Suggested change

	Because `FuturesUnordered` implements the `Stream` trait using `.next()`
	on it will return a `Future` which returns the return value from one of
	the futures inside of an `Option`.
	It will return `None` when all futures are completed.
	`FuturesUnordered` implements a `Stream` trait over the
	output of the futures in any order.

The example implies how to use that Stream (and hopefully we have dedicated docs for that too).

[capnhawkbill]

Yes I agree this is a lot clearer