doc: Create SECURITY.md

Author: Krinkle

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+The latest release is supported with security updates.
+
+## Reporting a vulnerability
+
+If you discover a vulnerability, we would like to know about it so we can take steps to address it as quickly as possible.
+
+E-mail your findings to security@jquery.com. Thanks!