starting dev branch

Author: martin-henz

README.md

@@ -91,3 +91,8 @@ Where there is a conflict between the two, the first one (lexmag) shall be the o
 Generated with [DBeaver](https://dbeaver.io/) on 8 September 2019
 
 ![Entity-Relationship Diagram for cadet](schema.png)
+
+## Source Academy 2021 development
+
+We will use this branch `sa_2021` for the development of the backend of Source
+Academy 2021. Please only target this branch in your SA-2021-related PRs.