update README

README.md

@@ -95,6 +95,19 @@ socket.emit("foo", "woot", new Ack() {
 });
 ```
 
+And vice versa:
+
+```java
+// ack from client to server
+socket.on("foo", new Emitter.Listener() {
+  @Override
+  public void call(Object... args) {
+    Ack ack = (Ack) args[args.length - 1];
+    ack.call();
+  }
+});
+```
+
 Use custom SSL settings:
 
 ```java