Add CRAN status badge

Author: andrie

README.md

@@ -8,6 +8,8 @@ release:
 [![Build Status](https://travis-ci.org/RevolutionAnalytics/checkpoint.svg?branch=release)](https://travis-ci.org/RevolutionAnalytics/checkpoint)
 dev: [![Build Status](https://travis-ci.org/RevolutionAnalytics/checkpoint.svg?branch=dev)](https://travis-ci.org/RevolutionAnalytics/checkpoint)
 
+**Version on CRAN**
+
 [![CRAN_Status_Badge](http://www.r-pkg.org/badges/version/checkpoint)](http://cran.r-project.org/package=checkpoint)
 
 ## Overview