Update README.md

Author: goddino

README.md

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-# Millicast SDK for Flutter
+# Millicast SDK for Flutter (beta)
 
-Flutter SDK for building a realtime broadcaster using the Millicast platform.
+Flutter SDK (currently under beta testing) for building a realtime broadcaster using the Millicast platform.
 This Software Development Kit (SDK) for Flutter allows developers to simplify Millicast services integration into their own Android and iOS apps.
 
 ## Table of Contents