Cookbook additions
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)

merged via https://github.com/pydata/pandas/commit/3b13646701951cb82c83d19a26743732e501abbb

thanks!
 http://pandas-docs.github.io/pandas-docs-travis/cookbook.html#plotting

thanks!

any other cookbook recipies you want to expand inline - go for it!
