Improved write-up (above and beyond man pages) for composite futures

[SteVwonder]

In a ☕ hour call, @garlick suggested that the complexity of the composite futures is a bit much to fully explain in a manpage and that it would be good to have a lengthier writeup on composite futures in ReST form, with inline code examples and the whole shebang.

[garlick]

Futures too, really. I was just rereading flux_future_get(3). I imagine it would be mystifying if you didn't already have a good understanding of the flux reactor.

It seems like a general introduction to the reactor, RPCs, and futures, with examples, could be useful to tie things together. Then chapter two: composite futures :-)