Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only certain branches are supported with security updates.
+
+| Version (branch) | Supported |
+| ---------------- | --------- |
+| main    | :white_check_mark: |
+| other   | :x:                |
+
+When using this code or reporting vulnerability please be sure to use supported branches and the most recent release tag.
+
+## Reporting a Vulnerability
+
+Use the `Report a vulnerability` link at https://github.com/GSA-TTS/terraform-cloudgov/security to report a security vulnerability
+on a supported branch of this repository.