README update for automatic page tracking caveat

Thanks for providing this great Angular module. It saved me a lot of time.

One little caveat which took me a while to figure out is that `Analytics` needs to be injected at least once for the Google Analytics script to be loaded and the automatic page tracking event handler to be activated. As I was not going to use `Analytics` in any of my controllers, I had to inject it in the main `run()` block.

I've updated `README.md` with an example.