Add documentation badge
It's helpful to have a link to the docs from the github repo README.
<img src="https://github.com/tensorflow/cleverhans/blob/master/assets/logo.png?raw=true" alt="cleverhans logo">
[![Build Status](https://travis-ci.org/tensorflow/cleverhans.svg?branch=master)](https://travis-ci.org/tensorflow/cleverhans)
+[![Documentation Status](https://readthedocs.org/projects/cleverhans/badge/?version=latest)](https://cleverhans.readthedocs.io/en/latest/?badge=latest)
This repository contains the source code for CleverHans, a Python library to
benchmark machine learning systems' vulnerability to
