Please document in README how to create nodes.json

Hi.  I'm trying to get ffmap-d3 working with my small mesh, but am unable to figure out how to create the nodes.json file.  The output from batctl vd json was not accepted as I
had hoped, and there is no clue in the README how to continue.  Perhaps a link to the http://www.nilsschneider.net/2013/02/03/ffmap-tutorial.html page could be included in the README?
