Add a SECURITY.md

Author: bbhtt

SECURITY.md

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+# Security policy for Flatpak builder
+
+## Supported versions
+
+Please see the [readme](https://github.com/flatpak/flatpak-builder/tree/main?tab=readme-ov-file#supported-versions).
+
+## Reporting vulnerabilities
+
+Please DO NOT report security issues via public mediums such as
+GitHub issues, forums, public mailing lists or Matrix.
+
+Please report all security issues via an email to the private mailing list 
+[[email protected]](mailto:[email protected]).
+
+## Security announcements
+
+The [[email protected]](mailto:[email protected])
+email list is used for messages about Flatpak security announcements.
+You can join the list [here](https://lists.freedesktop.org/mailman/listinfo/flatpak).
+Security issues may also be announced via GitHub security advisories.