Add RTD badge in README

Author: singingwolfboy

README.md

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-# Flask-Misaka [![Build Status](https://travis-ci.org/singingwolfboy/flask-misaka.png)](https://travis-ci.org/singingwolfboy/flask-misaka)  [![Coverage Status](http://codecov.io/github/singingwolfboy/flask-misaka/coverage.svg?branch=master)](http://codecov.io/github/singingwolfboy/flask-misaka?branch=master)
+# Flask-Misaka [![Build Status](https://travis-ci.org/singingwolfboy/flask-misaka.png)](https://travis-ci.org/singingwolfboy/flask-misaka) [![Coverage Status](http://codecov.io/github/singingwolfboy/flask-misaka/coverage.svg?branch=master)](http://codecov.io/github/singingwolfboy/flask-misaka?branch=master) [![Documentation Status](https://readthedocs.org/projects/flask-misaka/badge/?version=latest)](http://flask-misaka.readthedocs.org/en/latest/?badge=latest)
 
 A simple Flask extension to integrate the [Misaka](http://misaka.61924.nl)
 module, which is a Python interface to the excellent