Add example of using repeat() to add vertices

[krlawrence]

Add an example showing how repeat steps can be used to create vertices. Useful when testing.

gremlin> g.inject(1).repeat(addV('test').property(id,loops())).times(5)
==>v[4]
gremlin> g.V().valueMap(true)
==>[id:0,label:test]
==>[id:1,label:test]
==>[id:2,label:test]
==>[id:3,label:test]
==>[id:4,label:test]