document requirement for shared cache (see #141)

docs/source/include/quickstart_as_app/settings.py

@@ -22,10 +22,20 @@
     },
 }
 
+# If you use ACMEv2 or Celery, you must configure a cache which shares data
+# between different processes (e.g. between different uWSGI worker processes).
+CACHES = {
+    "default": {
+        "BACKEND": "django.core.cache.backends.redis.RedisCache",
+        "LOCATION": "redis://127.0.0.1:6379",
+        "OPTIONS": {"db": 1},
+    }
+}
+
 # The hostname used by default for URLs in certificates. Your Django project
 # should be available under this URL using HTTP (see below). If you use
 # ACMEv2, you will also need HTTPS.
 CA_DEFAULT_HOSTNAME = "ca.example.com"
 
-# RECOMMENDED: Use Celery as an asynchronous task worker if configured
+# RECOMMENDED: Use Celery as an asynchronous task worker
 CELERY_BROKER_URL = "redis://127.0.0.1:6379/0"

docs/source/quickstart/as_app.rst

@@ -17,6 +17,9 @@ Required software
 
 You do not need any special software besides Python |minimum-python| or later and a recent version of pip.
 
+If you want to use ACMEv2 or Celery, you need a cache that shares data between individual processes. From the
+backends included in Django, Memcached and Redis do this. This document configures Redis as a cache.
+
 It is strongly recommended that you run a `Celery task queue <https://docs.celeryproject.org/>`_. If you do,
 you need a message transport like RabbitMQ or Redis. Redis is used in the examples below, because it is
 easiest to set up and doubles as a cache.
@@ -84,6 +87,12 @@ You also need to include the URLs in your main :file:`urls.py`:
 .. literalinclude:: /include/quickstart_as_app/urls.py
    :language: python
 
+You can verify (some) aspects of the setup using Django system checks:
+
+.. code-block:: console
+
+   user@host:~$ python manage.py check --deploy
+
 Finally, invoke the regular :command:`manage.py` commands when you add new apps:
 
 .. code-block:: console