Add SECURITY.md

Based on template from Anne.
Fixes #165

SECURITY.md

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+# Reporting Security Issues
+
+To report a security issue, please email
+[[email protected]](mailto:[email protected])
+with a description of the issue, the steps you took to create the issue,
+affected versions, and, if known, mitigations for the issue.
+
+Our vulnerability management team will respond within 3 working days of your
+email. If the issue is confirmed as a vulnerability, we will open a
+Security Advisory and acknowledge your contributions as part of it. This project
+follows a 90 day disclosure timeline.