Add security policy

SECURITY.md

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+# Reporting security vulnerabilities
+
+Your efforts to responsibly disclose your findings are appreciated.
+
+** **Please do _not_ report security vulnerabilities through public GitHub issues.** **
+
+If you believe you have found a **security vulnerability**, then please report it to [email protected] so 
+your findings can be investigated, and if confirmed, fixed and released in a timely manner.
+
+Your report should include:
+
+- Mailpit version
+- A vulnerability description
+- Reproduction steps (if applicable)
+- Any other details you think are likely to be important
+
+You should receive an initial acknowledgement within 24 hours in most cases, and will kept updated throughout the process. 
+
+With your consent, your contributions will be publicly acknowledged.