Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version  | Supported          |
+| -------- | ------------------ |
+| 0.20.x   | :white_check_mark: |
+| < 0.20.x | :x:                |
+
+## Reporting a Vulnerability
+
+Please report security issues by e-mail to jelmer@jelmer.uk, ideally PGP encrypted to the key at https://jelmer.uk/D729A457.asc