Remove ReadTheDocs.org badge from README

According to the docs, it is no longer hosted there.

README.md

@@ -5,7 +5,6 @@
 [![Coverage Status](https://coveralls.io/repos/github/NodeBB/NodeBB/badge.svg?branch=master)](https://coveralls.io/github/NodeBB/NodeBB?branch=master)
 [![Dependency Status](https://david-dm.org/nodebb/nodebb.svg?path=install)](https://david-dm.org/nodebb/nodebb?path=install)
 [![Code Climate](https://codeclimate.com/github/NodeBB/NodeBB/badges/gpa.svg)](https://codeclimate.com/github/NodeBB/NodeBB)
-[![Documentation Status](https://readthedocs.org/projects/nodebb/badge/?version=latest)](https://readthedocs.org/projects/nodebb/?badge=latest)
 
 [**NodeBB Forum Software**](https://nodebb.org) is powered by Node.js and built on either a Redis or MongoDB database. It utilizes web sockets for instant interactions and real-time notifications. NodeBB has many modern features out of the box such as social network integration and streaming discussions, while still making sure to be compatible with older browsers.