docs: add security policy (#36873)

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you believe you have found a security vulnerability in Dify, please report it privately through GitHub Security Advisories:
+
+https://github.com/langgenius/dify/security/advisories/new
+
+Please do not report security vulnerabilities through public GitHub issues, discussions, or pull requests.
+
+When submitting a report, include as much relevant information as you can safely provide, such as:
+
+- A description of the vulnerability
+- Steps to reproduce, if safe to share privately
+- Affected components, versions, or configurations
+- Potential impact
+- Any suggested mitigation or fix, if available
+
+The maintainers will review reports submitted through GitHub Security Advisories and coordinate follow-up there.
+
+## Public Disclosure
+
+Please avoid publicly disclosing details of a vulnerability until it has been reviewed and, where appropriate, a fix or mitigation has been made available.
+
+## Security Updates
+
+Security fixes may be released through normal project releases or other appropriate channels. Users are encouraged to keep Dify deployments up to date.