Added callback for delivery & read receipts

README.md

@@ -33,6 +33,9 @@ Baileys is super easy to use:
     ``` javascript 
         client.handlers.presenceUpdated = (id, presence) => { /* called when you recieve an update on someone's presence */ } 
     ```
+    ``` javascript 
+        client.handlers.onMessageStatusChanged = (id, messageID, status) => { /* called when your message gets delivered or read */ } 
+    ```
     ``` javascript 
         client.handlers.onDisconnect = () => { /* called when internet gets disconnected */ } 
     ```