Create NOTICE.md

Author: shravan-sinch

NOTICE.md

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+## 🚨 Repository Deprecated – No Longer Maintained
+
+> **Important:** This SDK is no longer maintained.
+
+This repository (`Messages C# SDK`) has not been actively maintained for several years and is now officially **deprecated**. No further updates, bug fixes, or support will be provided.
+
+If you're building applications that integrate with the Sinch MessageMedia Messaging API, we recommend using direct REST API calls instead. You can find the complete and up-to-date API documentation here:  
+👉 [MessageMedia Messaging REST API Docs](https://messagemedia.github.io/documentation/#tag/Messages)
+
+The source code remains available under the [Apache 2.0 License](https://www.apache.org/licenses/LICENSE-2.0). Feel free to fork and update it to suit your needs.