Update README.md

README.md

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 Agora.io provides building blocks for you to add real-time voice and video communications through a simple and powerful SDK. You can integrate the Agora SDK to enable real-time communications in your own application quickly.
 
+## NOTE
+The `main` branch is major update base on the Agora Native SDK 4.x, which introduces some break changes. If you want to get the legacy version(the version < 6.0.0), please visit the [master](https://github.com/AgoraIO-Extensions/Agora-Flutter-SDK/tree/master) branch for reference.
+
 ## Usage
 
 To use this plugin, please add `agora_rtc_engine` as a dependency to