Merge pull request #3 from erminas/requirements_readme

Update requirements, raspbian stretch is required

README.md

@@ -7,6 +7,9 @@ Server
 The server is needed to communicate between the Node-RED RevPi nodes and the I/O pins on the RevPi. 
 It is a Python based websocket server which utilizes the Python library [RevPiModIO](https://revpimodio.org/) to interface between the RevPi process image and Node-RED. The associated RevPi nodes can be acquired via the [node-red-contrib-revpi-nodes package](https://flows.nodered.org/node/node-red-contrib-revpi-nodes) in the Node-RED Library or [here](https://github.com/erminas/node-red-contrib-revpi-nodes).
 
+### Requirements
+The server requires  [Raspbian Stretch ](https://revolution.kunbus.de/shop/de/stretch) for correct function.
+
 ### Installation
 1. Install the server with the following command:
 ```