docs: add SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We only support updates to the 2.x versions of Postal.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.x.x   | :white_check_mark: |
+| < 2.0   | :x:                |
+
+## Reporting a Vulnerability
+
+If you discover a vulnerability in Postal, please do not post an issue on GitHub. Instead you should send an
+e-mail to security@postalserver.io with details. We will get back to you directly.