Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest version of GitPython can receive security updates. If a vulnerability is discovered, a fix can be issued in a new release, while older releases
+are likely to be yanked.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 3.x.x   | :white_check_mark: |
+| < 3.0   | :x:                |
+
+## Reporting a Vulnerability
+
+Please report private portions of a vulnerability to sebastian.thiel@icloud.com that would help to reproduce and fix it. To receive updates on progress and provide
+general information to the public, you can create an issue [on the issue tracker](https://github.com/gitpython-developers/GitPython/issues).