Add a SECURITY.md policy

SECURITY.md

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+Thanks for helping make RomM safer for everyone.
+
+## Reporting Security Issues
+
+If you believe you have found a security vulnerability in RomM, please report it to us through coordinated disclosure.
+
+**Do not report security vulnerabilities through public GitHub issues, discussions, pull requests, or on our public Discord server.**
+
+Instead, use the [vulnerability report form](https://github.com/zurdi15/romm/security/advisories/new) on GitHub.
+
+Please include as much of the information listed below as you can to help us better understand and resolve the issue:
+
+- The type of issue (e.g., permission bypass, remote code execution, etc.)
+- Full paths of source file(s) related to the manifestation of the issue
+- The location of the affected source code (tag/branch/commit or direct URL)
+- Any special configuration required to reproduce the issue
+- Step-by-step instructions to reproduce the issue
+- Proof-of-concept or exploit code (if possible)
+- Impact of the issue (including how an attacker might exploit the issue)
+
+This information will help us investigate and patch the issue more quickly.