doc: add setup step to manual run instructions

README.md

@@ -244,6 +244,8 @@ pip install gunicorn
 
 source venv/bin/activate
 
+cp data/webpush.db.empty data/webpush.db
+
 # 3 workers would trigger 3 daily subscription runs
 gunicorn -b 0.0.0.0:3000 --log-level=debug --workers=1 app:app