add celery example

examples/celery/README.md

@@ -0,0 +1,27 @@
+Background Tasks with Celery
+============================
+
+This example shows how to configure Celery with Flask, how to set up an API for
+submitting tasks and polling results, and how to use that API with JavaScript. See
+[Flask's documentation about Celery](https://flask.palletsprojects.com/patterns/celery/).
+
+From this directory, create a virtualenv and install the application into it. Then run a
+Celery worker.
+
+```shell
+$ python3 -m venv .venv
+$ . ./.venv/bin/activate
+$ pip install -r requirements.txt && pip install -e .
+$ celery -A make_celery worker --loglevel INFO
+```
+
+In a separate terminal, activate the virtualenv and run the Flask development server.
+
+```shell
+$ . ./.venv/bin/activate
+$ flask -A task_app --debug run
+```
+
+Go to http://localhost:5000/ and use the forms to submit tasks. You can see the polling
+requests in the browser dev tools and the Flask logs. You can see the tasks submitting
+and completing in the Celery logs.