Add docs for explicitly closing Phantom when done.

I've updated now the README now to match example.js. 

I've also thought of what could be a nicer solution: 

You could provide your own 'pipe' method. It would add the 'finish' handler to the incoming stream, and then pass it onto the the normal 'pipe' command. 

So, the code would look the same as it did before the explicitly handlers were added.