Documentation page for topology definition YAML syntax

I hope to have included all available options.
I apologize in advance for any typo!
Since pyleus deps msgpack now, and we already dep argparse for python <2.7, I think we should just add a dep on simplejson in the same manner.

We could import it in the newly-added compat.py so we don't have to litter `try: import simplejson as json` everywhere.