docs: add example for multiple networks

README.md

@@ -264,6 +264,12 @@ Set the network to attach after the service container is started:
 dokku mysql:set lollipop post-create-network custom-network
 ```
 
+Set multiple networks:
+
+```shell
+dokku mysql:set lollipop post-create-network custom-network,other-network
+```
+
 Unset the post-create-network value:
 
 ```shell