Add change log

CHANGELOG.md

+# Changelog
+
+## 2.0
+ARSnova 2.0 has been in development for more than two years. Further releases
+can be expected much more frequently.
+
+This is actually the first major release of ARSnova Backend. It is called 2.0 to
+feature API compatibility with the simultaneously released ARSnova Mobile 2.0.
+
+**This version is brought to you by:**  
+Project management: Klaus Quibeldey-Cirkel  
+Lead programming: Andreas Gärtner, Daniel Gerhardt, Christoph Thelen,
+Paul-Christian Volkmer  
+Contributions: Sören Gutzeit, Julian Hochstetter, Jan Kammer, Daniel Knapp,
+Felix Schmidt, Artjom Siebert, Daniel Vogel  
+Testing & Feedback: Kevin Atkins, Kathrin Jäger  
+Sponsoring: [AG QLS](https://www.thm.de/zqe/qmnetzwerk/agqls),
+[HMWK](https://wissenschaft.hessen.de/wissenschaft/it-neue-medien/kompetenznetz-e-learning-hessen),
+[@LLZ](http://llz.uni-halle.de/)