Add Celery documentation (and additional advice on Redis) to the Performance howto, since it isn't going to be a default part of the project template / VM

docs/howto/performance.rst

@@ -13,7 +13,7 @@ We have tried to minimise external dependencies for a working installation of Wa
 Cache
 -----
 
-We recommend `Redis <http://redis.io/>`_ as a fast, persistent cache. Install Redis through package manager and enable it as a cache backend::
+We recommend `Redis <http://redis.io/>`_ as a fast, persistent cache. Install Redis through your package manager (on Debian or Ubuntu: ``sudo apt-get install redis-server``), add ``django-redis-cache`` to your requirements.txt, and enable it as a cache backend::
 
 	CACHES = {
 	    'default': {
@@ -25,7 +25,22 @@ We recommend `Redis <http://redis.io/>`_ as a fast, persistent cache. Install Re
 	    }
 	}
 
-Without a persistent cache, Wagtail will recreate all compressable assets at each server start, e.g. when any files change under ```./manage.py runserver```.
+Without a persistent cache, Wagtail will recreate all compressable assets at each server start, e.g. when any files change under ``./manage.py runserver``.
+
+
+Sending emails in the background using Celery
+---------------------------------------------
+
+Various actions in the Wagtail admin backend can trigger notification emails - for example, submitting a page for moderation. In Wagtail's default configuration, these are sent as part of the page request/response cycle, which means that web server threads can get tied up for long periods if a large number of emails is being sent. To avoid this, Wagtail can be configured to do this as a background task, using `Celery <http://www.celeryproject.org/>`_ as a task queue. To install Celery, add ``django-celery`` to your requirements.txt. A sample configuration, using Redis as the queue backend, would look like::
+
+    import djcelery
+
+    djcelery.setup_loader()
+
+    CELERY_SEND_TASK_ERROR_EMAILS = True
+    BROKER_URL = 'redis://'
+
+See the Celery documentation for instructions on running the worker process in development or production.
 
 
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