Add a note regarding Redis on PythonAnywhere

Fix #3124

docs/deployment-on-pythonanywhere.rst

@@ -71,13 +71,14 @@ Add these exports
     export DJANGO_AWS_ACCESS_KEY_ID=
     export DJANGO_AWS_SECRET_ACCESS_KEY=
     export DJANGO_AWS_STORAGE_BUCKET_NAME=
-    export DATABASE_URL='<see below>'
+    export DATABASE_URL='<see Database setup section below>'
+    export REDIS_URL='<see Redis section below>'
 
 .. note:: The AWS details are not required if you're using whitenoise or the built-in pythonanywhere static files service, but you do need to set them to blank, as above.
 
 
-Database setup:
+Database setup
----------------
+--------------
 
 Go to the PythonAnywhere **Databases tab** and configure your database.
 
@@ -113,6 +114,13 @@ Now run the migration, and collectstatic:
     python manage.py createsuperuser
 
 
+Redis
+-----
+
+PythonAnywhere does NOT `offer a built-in solution <https://www.pythonanywhere.com/forums/topic/1666/>`_ for Redis, however the production setup from Cookiecutter Django uses Redis as cache and requires one.
+
+We recommend to signup to a separate service offering hosted Redis (e.g. `Redislab <https://redis.com/>`_) and use the URL they provide.
+
 
 Configure the PythonAnywhere Web Tab
 ------------------------------------