feat(docs): add phonebook sync feature to README

Add documentation for the new phonebook synchronization system in the
README. This includes an overview of the `webtop2phonebook.php` and
`pbook2webtop.php` scripts, environment variable setup, and manual
configuration steps for enabling the service and timer.

README.md

@@ -69,6 +69,42 @@ modify the contents of the `./templates` directory directly, as they will
 be overwritten during the next application update. Files under
 `./state/webtop.properties.d/` are retained and are also added to backup.
 
+## Phonebook feature
+
+The phonebook synchronization system allows you to manage contacts between
+**WebTop** and a centralized **Phonebook**. This feature is supported by
+scripts and timer-based systemd services for automation.
+
+### Synchronization Scripts Overview
+
+- **`webtop2phonebook.php`:**
+   - Extracts contacts from WebTop.
+   - Processes the contacts and removes pre-existing WebTop entries in the
+     phonebook.
+   - Inserts the cleaned and updated contacts into the phonebook database.
+
+- **`pbook2webtop.php`:**
+   - Extracts contacts from the phonebook database.
+   - Cleans up the WebTop contacts folder and replaces its contents with
+     updated contacts.
+
+Both scripts use environment variables (`PHONEBOOK_DB_HOST`,
+`PHONEBOOK_DB_PASSWORD`) for connecting to the databases. The sync process to
+run daily at 23:00.
+
+### Manual Configuration Steps
+
+Follow these steps to manually configure and use the phonebook synchronization:
+1. Set the environment variables for the phonebook database connection.
+  - **`PHONEBOOK_DB_HOST`**: The hostname or IP address of the phonebook
+    database, formatted as `host:port`.
+  - **`PHONEBOOK_DB_PASSWORD`**: The password for the phonebook database user.
+1. Enable and start the service and timer, eg.:
+  ```bash
+  runagent -m webtop1 systemctl --user enable phonebook.service
+  runagent -m webtop1 systemctl --user enable phonebook.timer
+  runagent -m webtop1 systemctl --user start phonebook.timer
+  ```
 ## Uninstall
 
 To uninstall the instance: