Update README.md

Author: uerceg

README.md

@@ -22,6 +22,7 @@ If your app is an app which uses web views you would like to use adjust tracking
       * [In-App Purchase verification](#iap-verification)
       * [Callback parameters](#callback-parameters)
       * [Partner parameters](#partner-parameters)
+      * [Callback identifier](#callback-id)
    * [Session parameters](#session-parameters)
       * [Session callback parameters](#session-callback-parameters)
       * [Session partner parameters](#session-partner-parameters)
@@ -373,6 +374,19 @@ ADJEvent *event = [ADJEvent eventWithEventToken:@"abc123"];
 
 You can read more about special partners and these integrations in our [guide to special partners][special-partners].
 
+### <a id="callback-id"></a>Callback identifier
+
+You can also add custom string identifier to each event you want to track. This identifier will later be reported in event success and/or event failure callbacks to enable you to keep track on which event was successfully tracked or not. You can set this identifier by calling the `setCallbackId` method on your `ADJEvent` instance:
+
+
+```objc
+ADJEvent *event = [ADJEvent eventWithEventToken:@"abc123"];
+
+[event setCallbackId:@"Your-Custom-Id"];
+
+[Adjust trackEvent:event];
+```
+
 ### <a id="session-parameters"></a>Session parameters
 
 Some parameters are saved to be sent in every event and session of the Adjust SDK. Once you have added any of these parameters, you don't need to add them every time, since they will be saved locally. If you add the same parameter twice, there will be no effect.
@@ -525,6 +539,7 @@ The delegate functions will be called after the SDK tries to send a package to t
 Both event response data objects contain:
 
 - `NSString eventToken` the event token, if the package tracked was an event.
+- `NSString callbackid` the custom defined callback ID set on event object.
 
 If any value is unavailable, it will default to `nil`.