added ack constants and brief example

example.js

@@ -193,6 +193,22 @@ client.on('message_revoke_me', async (msg) => {
     console.log(msg.body); // message before it was deleted.
 });
 
+client.on('message_ack', (msg, ack) => {
+    /*
+        == ACK VALUES ==
+        ACK_ERROR: -1
+        ACK_PENDING: 0
+        ACK_SERVER: 1
+        ACK_DEVICE: 2
+        ACK_READ: 3
+        ACK_PLAYED: 4
+    */
+
+    if(ack == 3) {
+        // The message was read
+    }
+});
+
 client.on('disconnected', (reason) => {
     console.log('Client was logged out', reason);
 });

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,
+};