update readme with docker usage

README.md

@@ -1,7 +1,22 @@
 # watchdog
 
+[![Docker Cloud Build Status](https://img.shields.io/docker/cloud/build/hackaburg/watchdog)](https://hub.docker.com/r/hackaburg/watchdog)
+[![MicroBadger Size](https://img.shields.io/microbadger/image-size/hackaburg/watchdog)](https://hub.docker.com/r/hackaburg/watchdog)
+[![MicroBadger Layers](https://img.shields.io/microbadger/layers/hackaburg/watchdog))](https://hub.docker.com/r/hackaburg/watchdog)
+
 If run inside a docker-compose setup, the watchdog will notify the configured Slack channel about its shutdown.
 
+## Usage
+
+To get the watchdog, simply pull the `hackaburg/watchdog` image from the Docker hub and pass in your Slack webhook url as an environment variable:
+
+```bash
+$ docker run \
+    --rm \
+    -it \
+    -e SLACK_WEBHOOK_URL=https://your-slack-webhook_url \
+    hackaburg/watchdog
+```
 
 ## License