Add security policy.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the most recent version of kaml is supported with security fixes. Older versions are not supported.
+
+Sample projects are provided as-is. While I will endeavour to fix any issues reported, they are intended as educational examples, not production-ready code.
+
+## Reporting a Vulnerability
+
+If you discover or suspect you have discovered a vulnerability, please report it through [GitHub's security vulnerability reporting tool](https://github.com/charleskorn/kaml/security/advisories).
+Please include a short description of the issue and steps on how to reproduce it.
+
+The issue will be investigated and fixed privately, then disclosed publicly once a fix is available.
+
+Anyone who reports a vulnerability will be acknowledged in the release notes and security advisory.