[firebase_messaging] Add Kotlin support in README.md.

[beshr-eldebuch]

Description

For new projects with Kotlin's support, the project only contains MainActivity.kt and not MainActivity.java, and as for now, README.md doesn't support it.

• ✅ Added Kotlin support in README.md.

Related Issues

issue database.

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[beshr-eldebuch]

@googlebot I signed it!

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CLAs look good, thanks!

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[amirulabu]

It would be better to show from and to for people not familiar with editing AndroidManifest.xml files.

Suggested change

	<application android:name=".Application" ...>
	<!-- FROM -->
	<application android:name="io.flutter.app.FlutterApplication" ... />
	<!-- TO -->
	<application android:name=".Application" ... />