add security document (#362)

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+The security of our network is a top priority. If you have discovered a security vulnerability within our repository, we appreciate your cooperation in disclosing it to us in a responsible manner.
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+Instead, we kindly ask that you email us directly at `security@bandprotocol.com`. We will review your report and may follow up to request additional information.
+
+### What to Include
+
+If possible, please provide as much of the following information as you can. This will help us understand the nature and scope of the potential issue:
+
+- A clear and concise description of the potential vulnerability.
+- Steps to reproduce the issue or a proof-of-concept (if possible).
+- Any relevant tools, versions, or dependencies.
+- Any possible mitigations or workarounds you have identified.
+
+
+Thank you for helping keep our BandChain safe!