Update docs/configuring-playbook-sygnal.md: add the common section "Troubleshooting"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-sygnal.md

@@ -100,3 +100,7 @@ The shortcut commands with the [`just` program](just.md) are also available: `ju
 To make use of your Sygnal installation, you'd need to build your own Matrix client application, which uses the same API keys (for [GCM/FCM](https://firebase.google.com/docs/cloud-messaging/)) and certificates (for [APNS](https://developer.apple.com/notifications/)) and is to your Sygnal URL endpoint (e.g. `https://sygnal.example.com`).
 
 Refer to Sygnal's [Notes for application developers](https://github.com/matrix-org/sygnal/blob/master/docs/applications.md) document.
+
+## Troubleshooting
+
+As with all other services, you can find the logs in [systemd-journald](https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html) by logging in to the server with SSH and running `journalctl -fu matrix-sygnal`.