Add note

.config/example.yml

@@ -8,6 +8,9 @@
 # Final accessible URL seen by a user.
 url: https://example.tld/
 
+# ONCE YOU HAVE STARTED THE INSTANCE, DO NOT CHANGE THE
+# URL SETTINGS AFTER THAT!
+
 #   ┌───────────────────────┐
 #───┘ Port and TLS settings └───────────────────────────────────