Adding a Security Policy that redirects to CSA's Vulnerability Reporting page

Author: Alami-Amine

SECURITY.md

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+# Security Policy
+
+Thank you for helping us keep the Matter SDK secure!
+
+## Reporting a Vulnerability
+
+To report a security vulnerability, please follow the instructions at
+[Report a Vulnerability | How to Report - CSA-IOT](https://csa-iot.org/vulnerability-reporting/).
+
+We appreciate responsible disclosure and will work to address any issues
+promptly.