docs(SECURITY): initial version (#92)

Closes #79

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Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Important security fixes will be back-ported to at least the two most recent versions.
+
+## Reporting a Vulnerability
+
+This repository uses GitHub's [private vulnerability reporting](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing-information-about-vulnerabilities/privately-reporting-a-security-vulnerability) feature.
+
+You can submit a vulnerability report at https://github.com/copilot-extensions/preview-sdk.js/security/advisories/new.