Add example of startingWith() used with where().by()

[krlawrence]

Add an example where a value found in one vertex is used to do startingWith searches across other vertices.

gremlin> g.addV('key').property('prefix','Dal')
==>v[60873]
gremlin> g.V().hasLabel('key').as('a').V().hasLabel('airport').where(startingWith('a')).by('city').by('prefix').
values('city')
==>Dallas
==>Dallas
==>Dalaman
==>Dalian
==>Dalcahue
==>Dalat
==>Dalanzadgad