[ci skip] Create Security document. (#1061)

Author: Kichura

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you discover a **security vulnerability**, please **do not** open a public issue. Instead, use one of the following **confidential contact methods**:
+
+* **Email:** [florian@florianreuth.de](mailto:florian@florianreuth.de)
+* **Discord Username:** `florianreuth`
+
+If your issue is **not security-related** (e.g., bug reports, feature requests, or usage questions), please use the [GitHub Issues page](https://github.com/ViaVersion/ViaFabricPlus/issues) or contact us via the [Discord server](https://discord.gg/viaversion).