Update spark readme for spark ui instructions

Instructions to open an extra port to be able to see the spark ui for monitoring execution status.

all-spark-notebook/README.md

@@ -220,6 +220,7 @@ You may customize the execution of the Docker container and the Notebook server
 * `-e GRANT_SUDO=yes` - Gives the `jovyan` user passwordless `sudo` capability. Useful for installing OS packages. **You should only enable `sudo` if you trust the user or if the container is running on an isolated host.**
 * `-v /some/host/folder/for/work:/home/jovyan/work` - Host mounts the default working directory on the host to preserve work even when the container is destroyed and recreated (e.g., during an upgrade).
 * `-v /some/host/folder/for/server.pem:/home/jovyan/.local/share/jupyter/notebook.pem` - Mounts a SSL certificate plus key for `USE_HTTPS`. Useful if you have a real certificate for the domain under which you are running the Notebook server.
+* `-p 4040:4040` - Opens the port for the [Spark Monitoring and Instrumentation UI](http://spark.apache.org/docs/latest/monitoring.html). Note every new spark context that is created is put onto an incrementing port (ie. 4040, 4041, 4042, etc.), and it might be necessary to open multiple ports. `docker run -d -p 8888:8888 -p 4040:4040 -p 4041:4041 jupyter/all-spark-notebook`
 
 ## Conda Environments