Add an example showing how to merge two valueMap results

[krlawrence]

Add an example near section 4.16 showing how to merge two valueMap results

For example

gremlin> g.V(3,4).valueMap().unfold().group().by(keys).by(select(values).unfold().fold())

==>[country:[US,US],code:[AUS,BNA],longest:[12250,11030],city:[Austin,Nashville],elev:[542,599],icao:[KAUS,KBNA],lon:[
-97.6698989868164,-86.6781997680664],type:[airport,airport],region:[US-TX,US-TN],runways:[2,4],lat:[30.1944999694824,3
6.1245002746582],desc:[Austin Bergstrom International Airport,Nashville International Airport]]