chore: add SECURITY.md (#7724)

* chore: add SECURITY.md

* Update SECURITY.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Yan <61414485+yanthomasdev@users.noreply.github.com>

* fix: switch to GitHub Security Advisories for reporting

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Yan <61414485+yanthomasdev@users.noreply.github.com>

Author: 3 people

SECURITY.md

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+# Security Policy
+
+Decap CMS takes security seriously. This document outlines our security policy, supported versions, and how to report security vulnerabilities.
+
+## Supported Versions
+
+Security updates are provided for:
+
+| Version | Status | Lifecycle |
+|---------|--------|-----------|
+| 3.x | ✅ Actively Supported | Current stable release |
+| 2.x (Netlify CMS) | ❌ Unsupported | Legacy - no updates |
+| 1.x (Netlify CMS) | ❌ Unsupported | Legacy - no updates |
+
+**Note:** Decap CMS was renamed from Netlify CMS in February 2023. Versions 1.x and 2.x are no longer maintained. We recommend upgrading to version 3.x for security updates and new features.
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability in Decap CMS, please report it **confidentially** through GitHub Security Advisories. This allows us to investigate and address the issue without exposing it to the public until a fix is ready.
+
+**Submit your report at:** https://github.com/decaporg/decap-cms/security/advisories/new
+
+### What NOT to Do
+
+- Do not open a public GitHub issue for the vulnerability
+- Do not post details on social media or public forums
+- Do not attempt to exploit the vulnerability beyond confirming it exists
+- Do not access data beyond what's necessary to demonstrate the issue
+
+## Response Timeline
+
+This project follows a 90-day disclosure timeline.
+
+## Security Practices
+
+- Dependabot is enabled for automated security update checks
+- All code changes are tested in CI, including linting
+- End-to-end tests provide coverage of critical functionality
+- All pull requests require code review before merging
+- Passwords are not stored by Decap CMS; authentication is delegated to providers
+
+## Known Limitations
+
+- This is a **community-maintained open-source project**, not a commercial product with dedicated security resources
+- Security depends on the stability and practices of underlying dependencies and backend providers
+- Some vulnerabilities in dependencies may not be immediately patchable if they break backwards compatibility
+- This is a project with a long history, and many legacy dependencies can't be updated without significant refactoring