Added a doc for how to run Docker w/ Mesos 0.20

@guenter @BenWhitehead @ssorallen @tnachen reviews welcome!
This is not the ideal place to bring this up, but it's the first time I've seen the two `container` formats side-by-side. The old format could have been supported in 0.7.0, translated to the new format; none of the keys overlap. Granted the translation might be strange, but it would be far better than outright breaking backwards compatibility:

image -> docker: {image}
options[-v] -> volumes[]
options[-e] -> env{}
options[-p] -> ports[] && requirePorts