docs: add SECURITY.md

Author: send

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest version is supported with security updates.
+Please always use the latest release.
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Instead, report a vulnerability through GitHub's security advisory feature at
+https://github.com/send/send.github.com/security/advisories/new.
+
+You can expect an initial response within a few days.