added ack constants and brief example

src/util/Constants.js

@@ -91,3 +91,17 @@ exports.WAState = {
     UNPAIRED: 'UNPAIRED',
     UNPAIRED_IDLE: 'UNPAIRED_IDLE'
 };
+
+/**
+ * Message ACK
+ * @readonly
+ * @enum {number}
+ */
+exports.MESSAGE_ACK = {
+    ACK_ERROR: -1,
+    ACK_PENDING: 0,
+    ACK_SERVER: 1,
+    ACK_DEVICE: 2,
+    ACK_READ: 3,
+    ACK_PLAYED: 4,
+};