Added an example of how to use cond

Feel free to reformat this but please accept this example of how to use cond.  The current docs are 100% accurate however, after reading them, it still wasn't obvious how to use the cond statement in the aggregation framework.  Googling only returned a single page that actually had an example on it.  An example like this would have been VERY helpful to me and hopefully others find this helpful as well.