added on_announcement trigger to media_player doc

components/media_player/index.rst

@@ -208,6 +208,21 @@ This trigger is activated every time the media player finishes playing.
         on_idle:
           - logger.log: "Playback finished!"
 
+.. _media_player-on_announcement_trigger:
+
+``media_player.on_announcement`` Trigger
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This trigger is activated every time the media player is announcing.
+
+.. code-block:: yaml
+
+    media_player:
+      - platform: i2s_audio  # or any other platform
+        # ...
+        on_announcement:
+          - logger.log: "Announcing!"
+
 .. _media_player-is_idle_condition:
 
 ``media_player.is_idle`` Condition