Add a docker badge (#496)

Add a docker badge :)

README.md

@@ -11,6 +11,7 @@ bridging, with one or two bugs cropping up.
 
 
 [![Build Status](https://travis-ci.org/Half-Shot/matrix-appservice-discord.svg?branch=develop)](https://travis-ci.org/Half-Shot/matrix-appservice-discord)
+[![Docker Automated build](https://img.shields.io/docker/builds/halfshot/matrix-appservice-discord.svg)](https://hub.docker.com/r/halfshot/matrix-appservice-discord)
 [![#discord:half-shot.uk](https://img.shields.io/matrix/discord:half-shot.uk.svg?server_fqdn=matrix.half-shot.uk&label=%23discord:half-shot.uk&logo=matrix)](https://matrix.to/#/#discord:half-shot.uk)
 
 ### PRs