Update README.md

Author: mCodex

README.md

@@ -12,6 +12,9 @@ Modern secure storage for React Native, powered by Nitro Modules. Version 6 ship
 > [!WARNING]
 > Version 6 drops Windows support. The module now targets iOS and Android only.
 
+> [!IMPORTANT]
+> This README tracks the in-progress v6 work on `main`. For the stable legacy release, switch to the `v5.x` branch.
+
 ## Table of contents
 
 - [🚀 Highlights](#-highlights)