[doc] Add screenshot of interface to README

Author: g3force

README.md

@@ -5,7 +5,9 @@
 
 # ssl-game-controller
 
-The [ssl-refbox](https://github.com/RoboCup-SSL/ssl-refbox) replacement that will be introduced at RoboCup 2019
+The [ssl-refbox](https://github.com/RoboCup-SSL/ssl-refbox) replacement that will be introduced at RoboCup 2019.
+
+![Screenshot of Interface](./doc/screenshot_interface.png)
 
 ## Usage
 If you just want to use this app, simply download the latest [release binary](https://github.com/RoboCup-SSL/ssl-game-controller/releases/latest). The binary is self-contained. No dependencies are required.