consider adding an example with n_rows != 8 to the README

[ikashnitsky]

Hi Liam, thanks for you work! It took me too long to figure out out how to use you package specifying a custom number of rows, not the default 8. The n_row parameter seems to exist in hrbrmstr/waffle geom_waffle but in your implementation it lives in waffle_iron. It would be nice to show an example with a custom number of rows, perhaps 10x10.