add SECURITY.md file with new email

Author: basantagoswami

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please report security vulnerabilities privately to **security@bullbitcoin.com**.
+
+Do not open a public issue for security reports.
+
+Include as much detail as possible (affected version, steps to reproduce, impact). We will acknowledge your report and keep you updated on the fix.