Create SECURITY.md

Closes #1961.

SECURITY.md

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+# Security Policy
+
+We recommend always using the latest versions of Handlebars and its official companion libraries to ensure your application remains as secure as possible.
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 5.0.x   | :white_check_mark: |
+| 4.7.x   | :white_check_mark: |
+| < 4.0   | :x:                |
+
+## Reporting a Vulnerability
+
+To report a vulnerability, please visit https://github.com/handlebars-lang/handlebars.js/security.