Add docs badge to README (SciTools#2684)

Author: lbdreyer

README.md

@@ -4,6 +4,7 @@ Iris
 [![Join the chat at https://gitter.im/SciTools/iris](https://badges.gitter.im/SciTools/iris.svg)](https://gitter.im/SciTools/iris?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://api.travis-ci.org/repositories/SciTools/iris.svg?branch=master)](https://travis-ci.org/SciTools/iris/branches)
 [![DOI](https://zenodo.org/badge/doi/10.5281/zenodo.51860.svg)](https://dx.doi.org/10.5281/zenodo.51860)
+[![Documentation for master branch ](https://img.shields.io/badge/docs-master-blue.svg)](https://scitools-docs.github.io/iris/master/index.html)
 
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