Update README.rst

DEPR warning.

README.rst

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+⛔️ WARNING
+==========
+
+This repository is under-maintained. We are not fixing bugs or developing new features for it. We hope to deprecate if officially in October of 2024. For updates, `follow along on the DEPR ticket <https://github.com/openedx/public-engineering/issues/214>`_.
+
+Although we have stopped integrating new contributions, we always appreciate security disclosures and patches sent to [email protected]
+
 Part of `edX code`__.
 
 __ http://code.edx.org/