add docker image size badge

README.md

@@ -1,7 +1,7 @@
 [![Build Status](https://travis-ci.org/peers/peerjs-server.png?branch=master)](https://travis-ci.org/peers/peerjs-server)
 [![npm version](https://badge.fury.io/js/peer.svg)](https://www.npmjs.com/package/peer)
 [![Downloads](https://img.shields.io/npm/dm/peer.svg)](https://www.npmjs.com/package/peer)
-
+[![Docker Image Size (latest semver)](https://img.shields.io/docker/image-size/peerjs/peerjs-server)](https://hub.docker.com/r/peerjs/peerjs-server)
 # PeerServer: A server for PeerJS #
 
 PeerServer helps broker connections between PeerJS clients. Data is not proxied through the server.