Update configuration.rst
Update configuration.rst

Adds documentation as to the function of "E-Mail Check Interval" and its interaction with a crontab.
rossp/django-helpdesk,gwasser/django-helpdesk,rossp/django-helpdesk,gwasser/django-helpdesk,rossp/django-helpdesk,rossp/django-helpdesk,django-helpdesk/django-helpdesk,gwasser/django-helpdesk,django-helpdesk/django-helpdesk,django-helpdesk/django-helpdesk,django-helpdesk/django-helpdesk,gwasser/django-helpdesk