Update README.md

Add OnPublishToClient description

README.md

@@ -378,6 +378,7 @@ The function signatures for all the hooks and `mqtt.Hook` interface can be found
 | OnUnsubscribe          | Called when a client unsubscribes from one or more filters. Allows packet modification.                                                                                                                                                                                                                    | 
 | OnUnsubscribed         | Called when a client successfully unsubscribes from one or more filters.                                                                                                                                                                                                                                   | 
 | OnPublish              | Called when a client publishes a message. Allows packet modification.                                                                                                                                                                                                                                      | 
+| OnSendToSubscriber     | Called when a message is about to be sent to the subscriber.                                                                                                                                                                                                                                | 
 | OnPublished            | Called when a client has published a message to subscribers.                                                                                                                                                                                                                                               | 
 | OnPublishDropped       | Called when a message to a client is dropped before delivery, such as if the client is taking too long to respond.                                                                                                                                                                                         | 
 | OnRetainMessage        | Called then a published message is retained.                                                                                                                                                                                                                                                               |