Update README with instructions for the wger.properties config file

README.md

@@ -16,10 +16,26 @@ Install flutter, all its dependencies and create a new virtual device:
 <https://flutter.dev/docs/get-started/install>
 
 ### 3
-Generate translation files with ``flutter gen-l10n``
+Create a new file ``wger.properties`` in ``android/app`` (next to build.gradle):
+
+```properties
+WGER_API_KEY=123456
+```
+
+To just run/develop the app it only needs to have any value for WGER_API_KEY, but
+you need a correct value if you want to register via the app. For this you need
+to allow (a probably dedicated) user on the wger server to register users in its
+behalf. For this, generate an API KEY by visiting <http://localhost:8000/de/user/api-key>
+on your local instance and then run ``python3 manage.py add-user-rest theusername``
+
+You can later list all the registered users with: ``python3 manage.py list-users-api``  
 
 
 ### 4
+Generate translation files with ``flutter gen-l10n``
+
+
+### 5
 Start the application with ``flutter run`` or use your IDE (please note that
 depending on how you run your emulator you will need to change the IP address
 of the server)