Added an example of monkey patching

[xarg]

I've spent quite a bit of time looking how to do that monkey patching when using a message_queue. Thought it might be helpful for someone else.

[miguelgrinberg]

Thanks!

I like the idea of showing the example, but I think showing only eventlet when both eventlet and gevent are mentioned in that section can be even more confusing for people who is gevent. I'll expand on your example and also show how to monkey patch with gevent before merging.

[xarg]

I thought about that, but I don't use gevent so I'm not sure how to add it. Thanks!

[miguelgrinberg]

I have added monkey patching instructions for both eventlet and gevent to the docs. Thanks!