Added an example update handler to the readme.

I've just spent two hours because of this, :-(
The references I've found online get its data from req.query.
Thanks!

http://jackhq.tumblr.com/post/16035106690/nano-v1-2-x-document-update-handler-support-v1-2-x
http://wiki.apache.org/couchdb/Document_Update_Handlers#Usage