make a note for the uninitiated in the readme as to what LUIGI_HOST should be for running locally

[ryantuck]

Was stoked to try out luigi with docker. Downloaded but couldn't quite figure out what LUIGI_HOST should be. Kept trying localhost:8082 to no avail.

Finally figured out that docker-machine ip yields the correct IP address if you're running this locally. Would be helpful to add this note to readme.

Thanks for this!