Revise security policy and supported versions

Updated the security policy to reflect supported versions and reporting process.

SECURITY.md

@@ -0,0 +1,12 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| > 3.18.0   | :white_check_mark: |
+| <= 2.0   | :x:                |
+
+## Reporting a Vulnerability
+
+Use Github to securely report the vulnerability.