docs(README): GH-53 polish announcement

Author: wesleyboar

README.md

@@ -1,3 +1,7 @@
 # DesignSafe Use Case Template
 
-To edit or add use cases, follow [DesignSafe-CI/DS-User-Guide:/user-guide/docs/usecases README](https://github.com/DesignSafe-CI/DS-User-Guide/tree/main/user-guide/docs/usecases#readme).
+> [!WARNING]
+> **Deprecated.** Content migrated to [DesignSafe-CI/DS-User-Guide](https://github.com/DesignSafe-CI/DS-User-Guide) at `/user-guide/docs/usecases`. To contribute, follow [its README](https://github.com/DesignSafe-CI/DS-User-Guide/tree/main/user-guide/docs/usecases#readme).
+
+> [!IMPORTANT]
+> If your content is not available or accurate there, please open a PR to migrate or update it.