🔒️(security) create SECURITY.md for vulnerability reporting policy

Added a security policy document outlining vulnerability reporting and disclosure procedures.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Security is very important to us.
+
+If you have any issue regarding security, please disclose the information responsibly by emailing us at security@suite.anct.gouv.fr and not by creating an issue on the repository.
+
+We appreciate your effort to make Messages more secure.
+
+## Vulnerability disclosure policy
+
+Working with security issues in an open source project can be challenging, as we are required to disclose potential problems that could be exploited by attackers. With this in mind, our security fix policy is as follows:
+
+1. The Maintainers team will handle the fix as usual (Pull Request,
+release).
+2. In the release notes, we will include the identification numbers from the
+GitHub Advisory Database (GHSA) and, if applicable, the Common Vulnerabilities
+and Exposures (CVE) identifier for the vulnerability.
+3. Once this grace period has passed, we will publish the vulnerability.
+
+By adhering to this security policy, we aim to address security concerns
+effectively and responsibly in our open source software project.