Create SECURITY.md

Create security policy document with basic info.

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The latest minor version published on the PyPI server includes the latest security updates.
+
+## Reporting a Vulnerability
+
+Please email us at [[email protected]](mailto:[email protected]) to report a vulnerability or alternatively [open an issue](https://github.com/unitaryfund/mitiq/issues/new?assignees=&labels=bug&projects=&template=bug_report.md&title=). In both cases Unitary Fund will be in touch to triage the issue and respond.