Added "id" as third param to example function in README

Key point for this commit is that "id" of the message can be leveraged as the third parameter for worker.on function. 
worker.on( "message", function( msg, next, id ){ 
// Example
use the "id" to delete the message once processed.
}

README.md

@@ -30,8 +30,10 @@ Helper to simply implement a worker [RSMQ ( Redis Simple Message Queue )](https:
   var RSMQWorker = require( "rsmq-worker" );
   var worker = new RSMQWorker( "myqueue" );
 
-  worker.on( "message", function( msg, next ){
+  worker.on( "message", function( msg, next, id ){
   	// process your message
+  	console.log("Message id : " + id);
+  	console.log(msg);
   	next()
   });