docs: include demo link at top of README

Author: pmbittner

README.md

@@ -3,6 +3,7 @@
 [![Documentation](https://img.shields.io/badge/Documentation-Read-purple)][documentation]
 [![GitHubPages](https://img.shields.io/badge/GitHub%20Pages-online-blue.svg?style=flat)][website]
 [![License](https://img.shields.io/badge/License-GNU%20LGPLv3-blue)](LICENSE.LGPL3)
+[![Demo Repository](https://img.shields.io/badge/Demo-Try-blue)][demo]
 
 # DiffDetective - Variability-Aware Source Code Differencing