Add maintenance status and Kiro CLI transition notice (#3524)

Author: shankara-n

README.md

@@ -1,5 +1,8 @@
 # Amazon Q CLI
 
+> [!IMPORTANT]
+> This open source project is no longer being actively maintained and will only receive critical security fixes. Amazon Q Developer CLI is now available as [Kiro CLI](https://kiro.dev/cli/), a closed-source product. For the latest features and updates, please use Kiro CLI.
+
 ## Installation
 
 - **macOS**: