docs: add systemd timer docs

docs/systemd-timers.md

@@ -0,0 +1,57 @@
+# Running CalendarSync periodically using systemd
+
+To run CalendarSync periodically / automatically on specific times using systemd, two files are necessary.
+
+- A [service unit](https://www.freedesktop.org/software/systemd/man/systemd.service.html) file, which we will call: `CalendarSync.service`
+- A [timer unit](https://www.freedesktop.org/software/systemd/man/systemd.timer.html) file, which we will call: `CalendarSync.timer`
+
+The following content should be placed under: `.config/systemd/user/CalendarSync.service`:
+
+```systemd
+[Unit]
+Description=Run CalendarSync
+
+[Service]
+ExecStart=/path/to/binary/calendarsync --config path/to/your/sync.yaml --storage-encryption-key $key
+```
+
+The following content should be placed under: `.config/systemd/user/CalendarSync.timer`:
+
+```systemd
+[Unit]
+Description=Run CalendarSync regularly
+
+[Timer]
+OnCalendar=Mon..Fri 09:30
+OnCalendar=Mon..Fri 11:40
+Unit=calendarsync.service
+
+[Install]
+WantedBy=default.target
+```
+
+For more information check the [systemd docs](https://www.freedesktop.org/software/systemd/man/systemd.timer.html) on timer unit configurations.
+
+
+After creation of the files, reload systemd using:
+
+```bash
+systemctl daemon-reload
+```
+
+and enable the service and timer:
+
+```bash
+systemctl enable --user CalendarSync.timer
+```
+
+check the status with:
+
+```bash
+systemctl status --user CalendarSync.timer
+# and
+systemctl list-timers --user
+```
+
+Now at the configured times CalendarSync should run and sync the events without
+further intervention 🤞