add example for registration_timeout

Author: sstrigler

ejabberd.yml.example

@@ -30,6 +30,10 @@ loglevel: info
 ## clients.
 auth_stored_password_types: [scram_sha1, scram_sha256, scram_sha512]
 
+## If you enable registration through either mod_invites or open registration
+## uncomment the line below to reduce the impact of potential abuse.
+#registration_timeout: 3600
+
 listen:
   -
     port: 5222