Swift snippet using the authCallback.

README.md

@@ -304,6 +304,61 @@ channel.presence.history { presencePage, error in
 }];
 ```
 
+### Using the `authCallback`
+
+A callback to call to obtain a signed token request.  
+`ARTClientOptions` and `ARTRealtime` objects can be instantiated as follow:
+
+**Swift**
+
+```swift
+let clientOptions = ARTClientOptions()
+clientOptions.authCallback = { params, callback in
+    getTokenRequestJSONFromYourServer(params) { json, error in
+        //handle error
+        let tokenParams = ARTTokenParams(clientId:json["clientId"])
+
+        let tokenRequest = ARTTokenRequest(tokenParams:tokenParams,
+                                               keyName:json["keyName"],
+                                                 nonce:json["nonce"],
+                                                   mac:json["mac"])
+        tokenRequest.clientId = json["clientId"]
+        tokenRequest.ttl = json["ttl"]
+        tokenRequest.capability = json["capability"]
+        tokenRequest.timestamp = NSDate(timeIntervalSince1970:(json["timestamp"] / 1000))
+
+        callback(tokenRequest, nil)
+    }
+}
+
+let client = ARTRealtime(options:clientOptions)
+```
+
+**Objective-C**
+
+```objective-c
+ARTClientOptions *clientOptions = [[ARTClientOptions alloc] init];
+clientOptions.authCallback = ^(ARTTokenParams *params, void(^callback)(id<ARTTokenDetailsCompatible>, NSError*)) {
+    [self getTokenRequestJSONFromYourServer:params completion:^(NSDictionary *json, NSError *error) {
+        //handle error
+        ARTTokenParams *tokenParams = [[ARTTokenParams alloc] initWithClientId:json[@"clientId"]];
+
+        ARTTokenRequest *tokenRequest = [[ARTTokenRequest alloc] initWithTokenParams:tokenParams
+                                                                             keyName:json[@"keyName"]
+                                                                               nonce:json[@"nonce"]
+                                                                                 mac:json[@"mac"]];
+        tokenRequest.clientId = json[@"clientId"];
+        tokenRequest.ttl = [json[@"ttl"] doubleValue];
+        tokenRequest.capability = json[@"capability"];
+        tokenRequest.timestamp = [NSDate dateWithTimeIntervalSince1970:[json[@"timestamp"] doubleValue] / 1000];
+
+        callback(tokenRequest, nil);
+    }];
+};
+
+ARTRealtime *client = [[ARTRealtime alloc] initWithOptions:clientOptions];
+```
+
 ## Using the REST API
 
 ### Introduction