docs: Missing Redis setup instructions in Celery example README

## Description The `examples/celery/README.md` does not mention that **Redis is required** as a message broker for Celery to work. A beginner following the instructions will successfully start the Celery worker but immediately see repeated connection errors like: ``` ERROR/MainProcess] consumer: Cannot connect to redis://localhost:6379//: Error 111 connecting to localhost:6379. Connection refused.. Trying again in 2.00 seconds... (1/100) ``` This is confusing because nothing in the README warns about this dependency. ## Steps to Reproduce 1. Follow the README instructions exactly 2. Run `celery -A make_celery worker --loglevel INFO` 3. Celery starts but immediately fails trying to connect to Redis 4. Nothing in the README explains why or how to fix it ## Expected Behavior The README should mention that Redis is required and provide setup instructions for it. ## Proposed Fix Add the following before the `celery` run command in `README.md`: ```rst Celery requires a message broker. Install and start Redis before running the worker: $ sudo apt install redis-server # Debian/Ubuntu $ sudo service redis start # Verify Redis is running: $ redis-cli ping # Expected output: PONG ``` ## Environment - OS: Ubuntu / Debian (Linux) - Flask version: latest (main branch) - Celery version: 5.6.2 ## Additional Notes This is a documentation-only change with no impact on code. It improves the onboarding experience for beginners on Debian/Ubuntu systems, which are the most common Linux distributions.