Added section on MongoClient connection pooling

Not sure where the best place for this is, but we see connection churning problems very frequently. Here's a comment on the topic: http://blog.mongolab.com/2013/11/deep-dive-into-connection-pooling/#comment-1287521415

and the blog post goes into more detail. Let me know if you would like me to go into more detail and/or make changes.

Cheers!