Update README.

Author: JimmyTai

README.md

@@ -71,13 +71,8 @@ ZendeskMessaging.show();
 The SDK needs to be initialized before using authentication methods !
 
 ```dart
-// Method 1
 final ZendeskLoginResponse result = await ZendeskMessaging.loginUser(jwt: "YOUR_JWT");
 await ZendeskMessaging.logoutUser();
-
-// Method 2 if you need callbacks
-await ZendeskMessaging.loginUserCallbacks(jwt: "YOUR_JWT", onSuccess: (id, externalId) => ..., onFailure: () => ...);
-await ZendeskMessaging.logoutUserCallbacks(onSuccess: () => ..., onFailure: () => ...);
 ```
 ### Check authentication state (optional)
 
@@ -125,16 +120,6 @@ Allows custom conversation tags to be set, adding contextual data about the conv
 // Note: This method does not affect conversation tags already applied to the conversation.
 ```
 
-### Global observer (optional)
-
-If you need to catch all events you can attach a global observer to the ZendeskMessaging.
-
-```dart
-ZendeskMessaging.setMessageHandler((type, args){
-    print("$type => $args");
-});
-```
-
 ## Known shortcomings
 - **Attachment file**：`Currently does not support.` The official said it will be launched in the future.