Add more flesh to readme.

README.md

@@ -9,3 +9,18 @@ It was originally developed together with Städt. Leibniz-Gymnasium Remscheid
 as a proprietary product. Five years after the school stole the original
 code base, BiscuIT-ng started as a complete re-implementation as
 well-designed, free and open source software.
+
+BiscuIT-ng is a platform based on Django, that provides central funstions
+and data structures that can be used by apps that are developed and provided
+seperately. The core can interact closely with the Debian Edu / Skolelinux
+system.
+
+## Core features
+
+TBA.
+
+## Official add-on apps
+
+| App name                             | Purpose                                             |
+| ------------------------------------ | --------------------------------------------------- |
+| [ExNoMo][BiscuIT-App-ExNoMo]         | Enter grades into SchILD-NRW (Externes Noten-Modul) |