Add build argument in documentation

Author: jaymoulin

README.md

@@ -58,7 +58,7 @@ curl -sSL "https://gist.githubusercontent.com/jaymoulin/e749a189511cd965f45919f2
 
 To build this image locally 
 ```
-docker build -t jaymoulin/rpi-plex .
+docker build --build-arg PMS_URL=<latest_url_for_synology.spk> -t jaymoulin/rpi-plex .
 ```