DOC: Cookbook example- boxplot split by quartiles of another variable

As suggested by Jeff on this [StackOverflow answer](http://stackoverflow.com/a/23233196/1222578), I'm adding an example of a boxplot that uses quartiles of a second variable as the `by` variable.

Jeff also suggested adding the example in-line, rather than just providing a link to StackOverflow, so that's what I've done for now. The resulting plot currently looks like this, I'm happy to tweak it to better get the idea across:

![Boxplot example](http://i.imgur.com/yQZMlGH.png)