Add security policy file

Author: AaronAtDuo

Diff for: SECURITY.md

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+Duo is committed to providing secure software to all our customers and users.  We take all security concerns seriously and ask that any disclosures be handled responsibly.
+
+# Security Policy
+
+## Reporting a Vulnerability
+**Please do not use Github issues or pull requests to report security vulnerabilities.**
+
+If you believe you have found a security vulnerability in Duo software, please follow our response process described at https://duo.com/support/security-and-reliability/security-response.