Add project report reminders documentation

Issue HyphaApp#3989

docs/setup/administrators/setup-notifications.md

@@ -2,10 +2,56 @@
 
 As mentioned in [External Integrations](../../getting-started/architecture.md#external-integrations), Hypha offers notifications via e-mail and/or Slack.
 
-## Email
+## Notification providers setup
+
+Hypha sends messages via configured providers such as email or Slack.
+
+### Email
 
 For information on configuring email notifications for Hypha, see the [E-mail section](configuration.md#e-mail-settings) of the configuration reference.
 
-## Slack
+### Slack
 
 For information on configuring Slack notifications for Hypha, see the [Slack section](configuration.md/#slack-settings) of the configuration reference.
+
+## Project report reminders
+
+To send reminders for upcoming due reports, configure the following three things.
+
+### Wagtail administration project settings for report reminders
+
+Visit Wagtail admin, click Projects -> Project Settings (or URL `/admin/settings/application_projects/projectsettings`).
+
+Scroll down to add "Report reminder frequency" configurations. Click "Add report reminder frequency" for each reminder.
+
+### Script to run `notify_report_due` command
+
+Create a script that runs the `django` manage command `notify_report_due`.
+
+For example,
+
+```shell
+#!/bin/bash
+
+APP_ROOT=/opt/hypha
+cd "${APP_ROOT}" || exit 1
+export DJANGO_SETTINGS_MODULE="hypha.settings.production"
+
+exec venv/hypha/bin/python3 manage.py notify_report_due
+```
+
+The example assumes Hypha is installed at `/opt/hypha`, uses a virtual environment `venv/hypha`, with production
+settings.  Adjust as needed for your environment and settings. Make sure that the settings has `SEND_MESSAGES = True`.
+
+### Mechanism to run the above script
+
+Create a regularly executing task that runs the above script.
+
+For example,
+
+```cron
+37 9 * * * /usr/local/bin/send-report-reminders.sh
+```
+
+The example assumes `cron`, that the shell script calling `notify_report_due` is in `/usr/local/bin` and is executable,
+and that the report reminders (when needed) should be sent at 9:37 am according to the system clock.