Improve docs on setting up cron job
This ticket was migrated from here: https://issues.civicrm.org/jira/browse/CRM-18703
CiviCRM now registers a System Error if no cron job is recorded. This makes people new to the system nervous. I recommend adding cron job setup to the configuration checklist so that people work through learning/configuring this in the system during initial setup and avoid seeing a "System Error" message.