Add security policy (#275)

LGTM

Author: nobodyiam

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you have apprehensions regarding SOFAStack's security or you discover vulnerability or potential threat, don’t hesitate to get in touch with us by dropping a mail at sofastack@antgroup.com.
+
+In the mail, specify the description of the issue or potential threat. You are also urged to recommend the way to reproduce and replicate the issue. The SOFAStack community will get back to you after assessing and analysing the findings.
+
+PLEASE PAY ATTENTION to report the security issue on the security email before disclosing it on public domain.