Update README.md

connecting to the docker daemon

Author: pferrel

README.md

@@ -4,6 +4,12 @@
 
 ## Starting up
 
+First connect to the docker daemon
+
+```
+export DOCKER_HOST=:3375
+```
+
 This repository contains a set of scripts and configuration files to run a [Apache Spark](https://spark.apache.org/) standalone cluster from [Docker](https://www.docker.io/) container.
 
 To start master, workers or shell (on the same docker host),  you can invoke the following commands: