docs: Explain postgres user for db creation

packages/docs/pages/contributing/development-setup.md

@@ -30,6 +30,10 @@ Create the development database in the backend folder.
 yarn db:create
 ```
 
+:::warning
+`yarn db:create` commands expect that you have the `postgres` superuser. If not, you can create a superuser called `postgres` manually or you can create the database manually by checking PostgreSQL-related default values from the [app config](https://github.com/automatisch/automatisch/blob/main/packages/backend/src/config/app.ts).
+:::
+
 Run the database migrations in the backend folder.
 
 ```bash