Update README with read-only container instruction

README.md

@@ -45,6 +45,17 @@ docker run -d -p 8080:8080 plantuml/plantuml-server:tomcat
 
 The server is now listing to [http://localhost:8080](http://localhost:8080).
 
+## Read-only container
+
+The jetty container supports read-only files system, you can run the read-only mode with:
+```
+docker run -d -p 8080:8080 --read-only -v /tmp/jetty plantuml/plantuml-server:jetty-readonly
+```
+
+This makes the container compatible with more restricted environment such as OpenShift, just make sure you mount a volume (can be ephemeral) on `/tmp/jetty`.
+
+## Change base URL
+
 To run plantuml using different base url, change the `docker-compose.yml` file:
 ~~~
 args: