add (short) documentation on celery worker

docs/installation.rst

@@ -46,6 +46,10 @@ Installation
 
   all the request sent to your virtual machine (ip 192.168.33.10 by default) on port 80 will be proxied to port 8000 by apache.
 
+* Start up one or more celery worker to process async tasks:
+  .. code-block:: bash
+     (venv)vagrant@precise32:~/treeherder-service$ python manage.py celery worker
+
 .. _project repo: https://github.com/mozilla/treeherder-service
 .. _Vagrant: http://downloads.vagrantup.com
 .. _Virtualbox: https://www.virtualbox.org