add RTD badge to readme

Author: whitews

README.md

@@ -6,6 +6,7 @@
 
 [![Build Status](https://travis-ci.com/whitews/FlowIO.svg?branch=master)](https://travis-ci.com/whitews/FlowIO)
 [![Coverage](https://codecov.io/gh/whitews/FlowIO/branch/master/graph/badge.svg)](https://codecov.io/gh/whitews/flowio)
+[![Documentation Status](https://readthedocs.org/projects/flowio/badge/?version=latest)](https://flowio.readthedocs.io/en/latest/?badge=latest)
 
 ## Overview