[mqtt.espmilighthub] Fix things do not come ONLINE if MQTT Client Status Topic not supplied. (#14877)

* Update readme and add new thing status message.


Signed-off-by: Matthew Skinner <matt@pcmus.com>

bundles/org.openhab.binding.mqtt.espmilighthub/README.md

@@ -42,6 +42,9 @@ Leave this blank.
 **mqtt_state_topic_pattern:**
 `milight/states/:device_id/:device_type/:group_id`
 
+**MQTT Client Status Topic:**
+`milight/status`
+
 **group_state_fields:**
 IMPORTANT: Make sure only the following are ticked: