Add description of periodic email reminders in Readme
Add description of periodic email reminders in Readme

- Explain which periodic email reminders are sent.
- Describe how cronjobs are configured for reminder checks.

ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder