Adding How to add R packages

Adding an R package is not obvious if learners have installed Anaconda. Not sure where this lesson might fall in the grand scheme of things, but adding the simple command-line example helps.

setup.md

@@ -178,7 +178,9 @@ you must install the software described below.
     We use [knitr][cran-knitr], [stringr][cran-stringr], and [checkpoint][cran-checkpoint]
     to format lessons written in R Markdown,
     so you will need to install these to build R lessons
-    (and this example lesson).
+    (and this example lesson). The best way to install these packages is to open an R window and type:
+    "> install.packages('package_name', dependencies = TRUE)" Substitute the 'package_name' with
+    either 'knitr', 'stringer', and then 'checkpoint'.
 
 If you want to run `bin/lesson_check.py` (which is invoked by `make lesson-check`)
 you will need Jekyll (so that you have its Markdown parser, which is called Kramdown)