docs: Add security policy [skip ci]

Clarified supported versions and reporting process.

Author: bartventer

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest release (`v0.x.y`)[^1] is supported for security updates.
+
+[^1]: This project is in active initial development. While generally stable for most use cases, breaking changes or rapid development may still occur.
+
+## Reporting a Vulnerability
+
+Please use the **"Report a vulnerability"** button in the GitHub Security tab to confidentially disclose any security issues.
+
+## Updates
+
+Security fixes and announcements will appear on [GitHub Releases](https://github.com/bartventer/httpcache/releases) and [Issues](https://github.com/bartventer/httpcache/issues).