Update README.md

Instruct the user to create a persistent volume with the right permissions

See https://github.com/lovasoa/whitebophir/issues/164

README.md

 If you use the [docker](https://www.docker.com/) containerization service, you can easily run WBO as a container. 
 An official docker image for WBO is hosted on dockerhub as [`lovasoa/wbo`](https://hub.docker.com/repository/docker/lovasoa/wbo).
 
-You can run it with the following bash command :
+You can run the following bash command to launch WBO on port 5001, while persisting the boards outside of docker:
 
+```bash
+mkdir wbo-boards # Create a directory that will contain your whiteboards
+chown -R 1000:1000 wbo-boards # Make this directory accessible to WBO
+docker run -it --publish 5001:8080 --volume "$(pwd)/wbo-boards:/opt/app/server-data" lovasoa/wbo:latest # run wbo
 ```
-docker run -it --publish 5001:8080 --volume "$(pwd)/wbo-boards:/opt/app/server-data" lovasoa/wbo:latest
-```
-
-This will run WBO :
- - on port 5001
- - persisting the user data in a folder named `wbo-boards` in the current working directory.
 
 You can then access WBO at `http://localhost:5001`.