Add SECURITY.md file

SECURITY.md

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+# Security
+
+If you find a significant vulnerability, or evidence of one,
+please report it privately.
+
+We prefer that you use the [GitHub mechanism for privately reporting a vulnerability](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability#privately-reporting-a-security-vulnerability). Under the
+main repository's security tab, under "Reporting", click
+Advisories, then click "Report a vulnerability" to open the advisory form.