Readme: add Tips :: Match/replace the whole `body`

I spent considerable time finding out why `(deftemplate ... {[:body] (content ..)})` does nothing and
would like to help other users avoid the same problem. (Though hopefully only few need it.)

For more info/background, see the blog post http://theholyjava.wordpress.com/2014/04/08/kioo-how-to-replace-the-whole-body/ (feedback warmly welcomed)