docs: Adding Security Policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+This project uses semantic versioning to continuously release updated versions. This model should allow users to quickly and seamlessly upgrade once new versions become available.
+
+At the moment, only the latest version will be supported with security updates.
+
+## Reporting a Vulnerability
+
+If you should discover a security vulnerability with a CVSS score higher than 6.9 ('High' or 'Critical'), I would like to ask you to report this privately and responsibly to frank@steilerdev.de. You should receive feedback on those inquiries within 10 business days.
+
+Any other lower security score should be following the [bug reporting process laid out in the contributing guidelines](https://github.com/steilerDev/icloud-photos-sync/blob/main/CONTRIBUTING.md#issue).