Adding SECURITY.md file

This is based on the one from OSSF Scorecard

SECURITY.md

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+# Reporting Security Issues
+
+To report a security issue, please email
+[lebrungrandt@ornl.gov](mailto:lebrungrandt@ornl.gov)
+and [crtrott@sandia.gov](mailto:crtrott@sandia.gov)
+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 5 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.