Add versionadded directive

Author: OskarStark

notifier/events.rst

@@ -4,6 +4,11 @@
 Using Events
 ============
 
+.. versionadded:: 5.4
+
+    The ``MessageEvent``, ``FailedMessageEvent`` and ``SentMessageEvent`` were
+    introduced in Symfony 5.4.
+
 The class:``...\\..\\Transport`` of the Notifier component allows you to optionally hook
 into the lifecycle via events.