Mention that the example depends on R

[rgaiacs]

To avoid:

$ make serve                     
Loading required package: knitr
Loading required package: stringr
Loading required package: checkpoint
Error in generate_md_episodes() : 
  The checkpoint package is required to build the lessons.
Calls: source ... withVisible -> eval -> eval -> generate_md_episodes
In addition: Warning message:
In library(package, lib.loc = lib.loc, character.only = TRUE, logical.return = TRUE,  :
  there is no package called ‘checkpoint’
Execution halted
make: *** [Makefile:84: lesson-rmd] Error 1

Probably include instructions to install the R packages.