docs(contributing): added note for creating SECRET_KEY (#23417)

docs/docs/contributing/local-backend.mdx

@@ -26,6 +26,8 @@ pip install -r requirements/testing.txt
 pip install -e .
 
 # Initialize the database
+# Note: For generating a SECRET_KEY if you haven't done already, you can use the command:
+# echo "SECRET_KEY='$(openssl rand -base64 42)'" | tee -a superset_config.py
 superset db upgrade
 
 # Create an admin user in your metadata database (use `admin` as username to be able to load the examples)