Add Security policy (SECURITY.md)

Author: sbernard31

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Currently, [GitHub security advisories](https://help.github.com/en/articles/managing-security-vulnerabilities-in-your-project) is not activated on [eclipse](https://www.eclipse.org/) project.
+
+To report a vulnerability, your need to open a [bugzilla ticket](https://bugs.eclipse.org/bugs/enter_bug.cgi?product=Community&component=Vulnerability+Reports&keywords=security&groups=Security_Advisories).
+
+For more details, please look at https://www.eclipse.org/security/.
+
+## Supported Versions
+
+There is no final [release of leshan for now](https://github.com/eclipse/leshan/wiki/Roadmap). So security fixes are applied in master branch and then available in next milestone release.