add badge for rdocs (Issue 410)

Author: giuseppec

README.md

@@ -1,6 +1,7 @@
 # R interface to [OpenML.org](http://www.openml.org/) 
 
 [![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
+[![Rdoc](http://www.rdocumentation.org/badges/version/OpenML)](http://www.rdocumentation.org/packages/OpenML)
 [![CRAN Status Badge](http://www.r-pkg.org/badges/version/OpenML)](http://cran.r-project.org/web/packages/OpenML)
 [![Build Status](https://travis-ci.org/openml/openml-r.svg?branch=master)](https://travis-ci.org/openml/openml-r)
 [![CRAN Downloads](http://cranlogs.r-pkg.org/badges/OpenML)](http://cran.rstudio.com/web/packages/OpenML/index.html)