Add note to have correct version of docker-compose

In our birthday party, many attendees had compose version <1.5 installed, which won't work with the new compose `yml` files used in this tutorial.

Author: MathiasRenner

README.md

@@ -46,7 +46,10 @@ There will be a self-paced beginners’ tutorial for attendees to learn Docker b
 ### Pre-tutorial preparation
 At the training, you will need to bring your own computer. Before you go to a birthday party training, there are some steps you should do some preparation to get your work environment ready. Here are the steps:
 
-1. For Linux users, we need you to install [Docker engine] (https://docs.docker.com/engine/installation/) and [Docker compose] (https://docs.docker.com/compose/install/)
+1. For Linux users, we need you to install [Docker engine] (https://docs.docker.com/engine/installation/) and [Docker compose] (https://docs.docker.com/compose/install/). Make sure you have Docker compose version 1.6 or higher by executing 
+
+   ```docker-compose version```
+
 1. For PC and Mac users we need you to install [Docker toolbox for Mac and Windows](https://www.docker.com/products/docker-toolbox) and use [Docker Machine] (https://docs.docker.com/machine/get-started/) to create a virtual machine to run your Docker containers. Once your machine is created and you have connected your shell to this new machine, you're ready to run Docker commands on this host.  If you're using Linux you can skip to the next step.
 1. If you're new to Docker, pre-pull the docker images for the very basic tutorial