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

@guenter @BenWhitehead @ssorallen @tnachen reviews welcome!
I'm not inclined toward translation since it involves re-implementing part of the Docker command line parsing logic.  However, it's not too late to modify the key's name (any better name for the new `container` come to mind?) to allow one to specify tasks using either the old style (`mesos.CommandInfo.ContainerInfo`) or the new (`mesos.ContainerInfo`).  This would be sufficient to maintain support for existing external containerizer implementations like Deimos.  On the other hand, it would be more surface area to explain, test, and maintain.  For reference, the original Issue and PR were #437 and #439.