Create SECURITY.md

Author: forman

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.4.x   | :white_check_mark: |
+| < 0.4   | :x:                |
+
+## Reporting a Vulnerability
+
+To report a vulnerability, please post and [issue](https://github.com/bcdev/zappend/issues)
+and use prefix `[SECURITY]` for the title. Security issues will be treated with high priority.