chore: Create SECURITY.md (#420)

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+The most-recently-released version of the library is supported with security updates.
+For example, if a security issue is discovered that affects an old version (e.g. 0.5.1)
+and the latest release (e.g. 0.6.7) a new 0.6.x patch will be released but a new 0.5.x
+patch release will not be made. You should plan to update to the latest version to
+receive any new features or bugfixes of any kind.
+
+## Reporting a Vulnerability
+
+To report a suspected security issue, you can contact [email protected] rather than opening
+a public issue.