docs: Add security policy (#202)

Author: pavelzw

SECURITY.md

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+# Reporting Security Issues
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+We take security bugs in our projects seriously. We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
+
+To report a security issue, please use the GitHub Security Advisory ["Report a Vulnerability"](https://github.com/prefix-dev/setup-pixi/security/advisories/new) tab.
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+We will send a response indicating the next steps in handling your report. After the initial reply to your report, the security team will keep you informed of the progress towards a fix and full announcement, and may ask for additional information or guidance.