docs: add note

Author: kazupon

README.md

@@ -6,6 +6,13 @@
 
 Internationalization middleware & utilities for h3 (and therefore also for Nitro, which is using h3)
 
+<!-- eslint-disable markdown/no-missing-label-refs -- NOTE(kazupon): ignore github alert -->
+
+> [!NOTE]
+> You’re viewing the main branch, which is developing for h3 v2. If you want to check h3 v1, please see the [v1 branch](https://github.com/intlify/h3/tree/v1)
+
+<!-- eslint-enable markdown/no-missing-label-refs -- NOTE(kazupon): ignore github alert -->
+
 ## 🌟 Features
 
 ✅️ &nbsp;**Translation:** Simple API like