Added RTD badge to README

README.md

@@ -1,4 +1,6 @@
 [![7.x-3.x Build Status](https://travis-ci.org/tripal/tripal.svg?branch=7.x-3.x)](https://travis-ci.org/tripal/tripal)
+[![Documentation Status](https://readthedocs.org/projects/tripal/badge/?version=latest)](https://tripal.readthedocs.io/en/latest/?badge=latest)
+
 [![DOI](https://zenodo.org/badge/42666405.svg)](https://zenodo.org/badge/latestdoi/42666405)