showing default domain & port in basic usage example

Hi Anders,

this week we had a support case in the ArangoDB group where someone tried "desperately" to get the client up and running - obviously very new to nodejs (like me ;-)). The basic example given in the documentation did work for me when I included the default domain & port. To make it a bit easier for other n00bs I suggest to add this information to the readme.
Feel free to discard this PR. :-)

Dorthe