doc: Update security policy

Author: jcdcdev

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The following table outlines the versions of the project that are currently supported with security & feature updates:
+
+> [!NOTE]
+> Once a version starts its security phase, it will no longer receive feature updates. Only critical bug fixes and security updates will be provided.
+
+| Package Version                                                                            | Umbraco Version | Security Phase Start | End of Life |
+| ---------------------------------------------------------------------------------- | --------------- | -------------------- | ----------- |
+| [15.x](https://github.com/jcdcdev/Umbraco.Community.SimpleTrees/tree/v15) | 15 | 2025-08-14 | 2025-11-14 |
+| [16.x](https://github.com/jcdcdev/Umbraco.Community.SimpleTrees/tree/v16) | 16 | 2026-03-12 | 2026-06-12 |
+
+
+## Future Support
+
+Project maintainers plan to support all STS (Short-Term Support) and LTS (Long-Term Support) versions of Umbraco. However, exact release dates cannot be guaranteed.
+
+> [!NOTE]
+> Visit [jcdc.dev/blog/umbraco-version-information](https://jcdc.dev/blog/umbraco-version-information) for more information on Umbraco versions.
+
+## Reporting a Vulnerability
+
+If you discover a vulnerability in this project, please follow one of these steps to report it:
+
+- Create an [issue](https://github.com/jcdcdev/Umbraco.Community.SimpleTrees/security/advisories/new)
+- Contact the project author privately at [jcdc.dev/contact](https://jcdc.dev/contact)
+
+### Details
+
+Include as much information as possible about the vulnerability, including:
+
+- Steps to reproduce
+- Potential impact
+- Any suggested fixes
+
+### Acknowledgment
+
+You will receive an acknowledgment of your report as soon as possible.
+
+> [!NOTE]
+> Response times may vary depending on other commitments.
+
+### Resolution
+
+Once the vulnerability is confirmed, project maintainers will work to resolve it as quickly as possible.
+
+You will be notified once the issue has been resolved or rejected.
+
+> [!TIP]
+> If the vulnerability is accepted, you will receive credit in the release notes.
+
+Thank you for helping to keep this project secure!
+