make docker compose example more clear in readme

README.md

@@ -34,6 +34,10 @@ $ gonic -h                                           # or see "configuration opt
 or else you can run in docker, available on dockerhub as `sentriz/gonic`
 
 ```yaml
+# example docker-compose.yml
+
+version: '3'
+services:
   gonic:
     image: sentriz/gonic:latest
     environment:
@@ -43,7 +47,7 @@ gonic:
     - 80
     volumes:
     - ./data:/data
-  - ${YOUR_MUSIC}:/music:ro
+    - /path/to/music:/music:ro
 ```
 
 ## configuration options