Update README.md

Author: beshr-eldebuch

packages/firebase_messaging/README.md

@@ -73,7 +73,8 @@ By default background messaging is not enabled. To handle messages in the backgr
 
    Note: you can find out what the latest version of the plugin is [here ("Cloud Messaging")](https://firebase.google.com/support/release-notes/android#latest_sdk_versions).
 
-1. Add an `Application.java` class to your app in the same directory as your `MainActivity.java`. This is typically found in `<app-name>/android/app/src/main/java/<app-organization-path>/`.
+#### Java's support:
+2. Add an `Application.java` class to your app in the same directory as your `MainActivity.java`. This is typically found in `<app-name>/android/app/src/main/java/<app-organization-path>/`.
 
    ```java
    package io.flutter.plugins.firebasemessagingexample;
@@ -98,19 +99,48 @@ By default background messaging is not enabled. To handle messages in the backgr
    }
    ```
 
-1. In `Application.java`, make sure to change `package io.flutter.plugins.firebasemessagingexample;` to your package's identifier. Your package's identifier should be something like `com.domain.myapplication`.
+3. In `Application.java`, make sure to change `package io.flutter.plugins.firebasemessagingexample;` to your package's identifier. Your package's identifier should be something like `com.domain.myapplication`.
 
    ```java
    package com.domain.myapplication;
    ```
+#### Kotlin's support:
+2. Add an `Application.kt` class to your app in the same directory as your `MainActivity.kt`. This is typically found in `<app-name>/android/app/src/main/kotlin/<app-organization-path>/`.
+```kotlin
+package io.flutter.plugins.firebasemessagingexample
+
+import io.flutter.app.FlutterApplication
+import io.flutter.plugin.common.PluginRegistry
+import io.flutter.plugin.common.PluginRegistry.PluginRegistrantCallback
+import io.flutter.plugins.GeneratedPluginRegistrant
+import io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService
+
+class Application : FlutterApplication(), PluginRegistrantCallback {
+
+    override fun onCreate() {
+        super.onCreate()
+        FlutterFirebaseMessagingService.setPluginRegistrant(this);
+    }
+
+    override fun registerWith(registry: PluginRegistry?) {
+        io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin.registerWith(registry?.registrarFor("io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin"));
+    }
+}
+```
+3. In `Application.kt`, make sure to change `package io.flutter.plugins.firebasemessagingexample` to your package's identifier. Your package's identifier should be something like `com.domain.myapplication`.
+
+   ```kotlin
+   package com.domain.myapplication;
+   ```
+---
 
-1. Set name property of application in `AndroidManifest.xml`. This is typically found in `<app-name>/android/app/src/main/`.
+4. Set name property of application in `AndroidManifest.xml`. This is typically found in `<app-name>/android/app/src/main/`.
 
    ```xml
    <application android:name=".Application" ...>
    ```
 
-1. Define a **TOP-LEVEL** or **STATIC** function to handle background messages
+5. Define a **TOP-LEVEL** or **STATIC** function to handle background messages
 
    ```dart
    Future<dynamic> myBackgroundMessageHandler(Map<String, dynamic> message) async {
@@ -131,7 +161,7 @@ By default background messaging is not enabled. To handle messages in the backgr
    Note: the protocol of `data` and `notification` are in line with the
    fields defined by a [RemoteMessage](https://firebase.google.com/docs/reference/android/com/google/firebase/messaging/RemoteMessage). 
 
-1. Set `onBackgroundMessage` handler when calling `configure`
+6. Set `onBackgroundMessage` handler when calling `configure`
 
    ```dart
    _firebaseMessaging.configure(