docs(contrib): Clarify source of DEV_REGISTRY IP Address

It was fairly unclear what the source of the `DEV_REGISTRY` IP address is. It isn't a problem on Mac OS X as you explicitly tell them the IP address with `boot2docker`, but on linux that is no help.
Reading this in context (finally--sorry!), I think this is better off *not* being a "Note:" callout section, just a sentence following the `make dev-registry` example. And maybe this wording is a bit more clear:
```
 ``make dev-registry`` prints the registry's address and port when using ``boot2docker``; otherwise, use your host's IP address as returned by ``ifconfig`` with port 5000 for ``DEV_REGISTRY``.
```