Create SECURITY.md

Author: connor-baer

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The following versions of this project are currently being supported with security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 2.x     | :white_check_mark: |
+| 1.x     | :x:                |
+
+## Reporting a Vulnerability
+
+Vulnerabilities can be [reported privately through GitHub](https://github.com/connor-baer/action-sync-branch/security/advisories/new). You can expect a report to be reviewed within 3 business days and a response outlining the next steps within 5 business days.