Added SECURITY.md

Added the security policy to include details on supported versions, reporting vulnerabilities, and response timelines.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Security fixes are provided for the latest published release of
+ghc::filesystem. Older releases are not normally maintained.
+
+The currently supported release is v1.5.16.
+
+Users should update to the latest release before reporting a vulnerability
+that might already have been fixed.
+
+## Reporting a Vulnerability
+
+Please do not open a public GitHub issue for a suspected security
+vulnerability.
+
+Use the **Report a vulnerability** button in the repository's
+[Security Advisories](https://github.com/gulrak/filesystem/security/advisories)
+section.
+
+If private vulnerability reporting is unavailable, contact
+s.schuemann@pobox.com with the subject "ghc::filesystem security report".
+
+Please include, where possible:
+
+- The affected ghc::filesystem version or commit.
+- The operating system, compiler, architecture, and language standard.
+- A description of the security impact and expected attack scenario.
+- Reproduction steps or a minimal proof of concept.
+- Whether untrusted input is required to trigger the problem.
+- Any known workarounds or suggested fixes.
+- Any planned public disclosure date, and whether and how you would like to be credited.
+
+Reports should contain enough information to distinguish a security
+vulnerability from an ordinary correctness or standards-conformance issue.
+
+## Scope
+
+Examples of potentially security-relevant issues include:
+
+- Memory corruption or undefined behavior reachable through untrusted input.
+- Path parsing or normalization errors that cause an operation to affect a
+  different filesystem location than specified by the API.
+- Symlink, reparse-point, or namespace handling that bypasses documented
+  behavior.
+- Encoding or argument-conversion bugs that cause memory-safety problems,
+  unintended filesystem operations, or practical denial of service.
+- Error-handling defects that result in invalid memory access or silently
+  performing an unsafe operation.
+
+Ordinary conformance differences, unsupported-platform problems, and
+application-level path traversal caused solely by passing unsanitized paths to
+filesystem operations should be reported as regular GitHub issues.
+
+ghc::filesystem provides filesystem primitives and does not provide sandboxing,
+access control, or validation of application-specific path policies.
+
+## Preferred Languages
+
+I prefer all communications to be in English, but German works too.
+
+## Response and Disclosure
+
+I will try to acknowledge reports within seven days. Investigation and release
+timelines depend on the affected platforms and the complexity of reproducing
+the issue.
+
+Please allow time to investigate and prepare a fix before public disclosure.
+When appropriate, the fix and disclosure will be coordinated through a GitHub
+Security Advisory. Reporters will be credited unless they prefer to remain
+anonymous.
+
+This project does not operate a paid bug bounty program and can not afford one.