Add Security Policy

SECURITY.md

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+# Security Policy
+
+This security policy applies to public projects under the [Kit organization](https://github.com/kit) on GitHub.
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability, please report via any of the below:
+
+- [Kit](https://kit.com/report-vulnerability)
+- [GitHub](https://github.com/Kit/convertkit-wordpress-libraries/security/advisories)
+- [Email](mailto:security@kit.com)
+
+Please do **not** report security issues publicly via GitHub issues or discussions. Security reports sent via the above channels will be acknowledged within 48 hours.
+
+## Security Disclosure Process
+
+When reporting a security vulnerability, please include:
+
+1. **Description** - A clear description of the vulnerability
+2. **Impact** - What kind of vulnerability it is and who it impacts
+3. **Reproduction** - Detailed steps to reproduce the issue
+4. **Proof of Concept** - If applicable, include proof-of-concept code
+5. **Suggested Fix** - If you have ideas for how to fix the issue
+
+## Security Response Timeline
+
+- **Initial Response**: Acknowledgement within 48 hours of receiving the report
+- **Investigation**: Investigation and validation of the reported vulnerability
+- **Fix Development**: Development of a patch or mitigation strategy
+- **Release**: Coordinated disclosure and release of security update
+- **Public Disclosure**: Public announcement after users have had time to upgrade