docs: add organization-wide SECURITY.md file

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you believe you have found a security vulnerability, please report it
+privately using one of the following channels:
+
+- GitHub's private vulnerability reporting: open the **Security and quality** tab
+  of the affected repository, then click **Report a vulnerability**.
+- Email: [security@hetzner.com](mailto:security@hetzner.com)
+
+Please do not report security vulnerabilities through public GitHub issues or discussions.
+
+We welcome all reports. If you are unsure whether an issue qualifies as
+a security vulnerability, please report it anyway. We will review the
+details and work with you to determine whether action is needed.
+
+When reporting, please include as much of the following as possible:
+
+- A description of the issue and its potential impact
+- Steps to reproduce, or a proof of concept
+- Affected versions, components, or configurations
+
+## Encrypted Communication
+
+We encourage you to encrypt reports that contain sensitive information. You can
+send OpenPGP-encrypted email to
+[security@hetzner.com](mailto:security@hetzner.com) using our public key,
+available at [https://hetzner.com/pgp-key.txt](https://hetzner.com/pgp-key.txt).