chore: set up security policy

PiperOrigin-RevId: 588939087

Author: asyncliz

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Security updates are only applied to the latest published version.
+
+## Reporting a Vulnerability
+
+Report vulnerabilities in a private
+[GitHub security advisory](https://github.com/material-components/material-web/security/advisories/new).
+
+**Please do not disclose security vulnerabilities in public issues.**