Add SECURITY.md (#460)

SECURITY.md

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+# Reporting Security Issues
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+**Please do not report security vulnerabilities through public GitHub issues.**
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+We at STACKIT take security seriously. We appreciate your efforts to responsibly disclose your findings, and will make every effort to acknowledge your contributions.
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+To report a security issue, please send an email to [stackit-security@stackit.de](mailto:stackit-security@stackit.de).
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+Our team 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.