miniobs example docs out of date

[BrianJKoopman]

We dropped crossbar from the requirements.txt in #173, which sort of breaks the miniobs example if followed from a fresh install. It should be made clear that crossbar needs to be installed before step 1, or that one should run a crossbar server in a Docker container (as the Sphinx docs instructs.) This likely will be made clear once #208 is merged, but just to track that this gets updated.