Include vignette to explain how to use checkpoint inside a markdown document

[andrie]

Use the solution suggested in #216

[andrie]

Version 0.3.20 contains a vignette explaining how to do this:

I have an example (badly written) vignette on gitbub that explains this approach: http://htmlpreview.github.io/?https://github.com/RevolutionAnalytics/checkpoint/blob/master/vignettes/using-checkpoint-with-knitr.html