Add link to Code School's Try R free course

Much like Try Git, Try R is a free, online crash course
on the R language. Very helpful.

Signed-off-by: Felipe Balbi <balbi@kernel.org>

Author: felipebalbi

rprog.md

@@ -11,4 +11,5 @@ permalink: /rprog/
 
 ## R Language
 
-- [Some notes on the R Language](http://lopezrj.github.io)
+- [Some notes on the R Language](http://lopezrj.github.io)
+- [Try R](http://tryr.codeschool.com/)