Update README with new function

Author: fbenevides

README.md

@@ -609,6 +609,20 @@ PusherChannel *myChannel = [pusher subscribeWithChannelName:@"my-channel"];
 
 This returns PusherChannel object, which events can be bound to.
 
+For non-presence channels, you can also provide a function that will be called when a client either subscribes or unsubscribes to a channel with the number of subscribers as a parameter. Also, this function is available as a parameter to `subscribe` function. 
+
+```swift
+let onSubscriptionCountChanged = { (count: Int) in
+    let message = "\(count) subscriptions"
+    self.message = message
+}
+
+let channel = pusher.subscribe(
+    channelName: "my-channel",
+    onSubscriptionCountChanged: onSubscriptionCountChanged
+)
+```
+
 ### Private channels
 
 Private channels are created in exactly the same way as public channels, except that they reside in the 'private-' namespace. This means prefixing the channel name: