Create SECURITY.rst

Using sample at aio-libs/.github

.github/SECURITY.rst

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+=========================
+Reporting Vulnerabilities
+=========================
+
+**⚠️ Please do not file public GitHub issues for security
+vulnerabilities as they are open for everyone to see! ⚠️**
+
+We encourage responsible disclosure practices for security vulnerabilities.
+
+If you believe you've found a security-related bug, email `Andrew Svetlov
+<mailto:andrew.svetlov+aio-libs-security@gmail.com>`_ and/or
+`Pandu POLUAN <mailto:pepoluan+aiosmtpd@gmail.com>`_
+instead of filing a ticket or posting to any public groups.
+We'll try to assess the problem and disclose it in a responsible way.