README does not display definition of nil

[David-Duffy]

Hi. For some reason, although README.Rmd has two examples, the first being "nil <- grid::nullGrob()", only the second (main) example actually appears in README.md and on the github page. Library otherwise great, btw!

[MHenderson]

Thanks for your feedback. Apologies for not spotting this issue had been raised sooner. I had deliberately hidden the definition of nil in the README because I was intending to define it in the package (hence it wouldn't also need to be defined in the README). But I forgot to go back and actually do that. I can see why it would be confusing to anyone reading the README. Will fix it in the next release. Thanks for pointing it out!