Update README.md

Deprecate the repository.

Author: seanlip

README.md

@@ -1,2 +1,10 @@
-# oppia-ml-proto
+# [DEPRECATED] oppia-ml-proto
+
+> [!CAUTION]  
+> The functionality in this repository is **deprecated**, and the corresponding code was removed from the main Oppia repository in https://github.com/oppia/oppia/pull/21356.
+>
+> This repository has been archived for reference purposes, and further development on this project is not expected.
+
+## Description
+
 The public API defined for Oppia-ML to be shared between Oppia-ML and other integrated platforms.