Add SECURITY.md "boilerplate"

Author: Vidas P

SECURITY.md

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+# Reporting Vulnerabilities
+
+**Please do not file GitHub issues for security vulnerabilities as they are
+public.**
+
+If you believe you have uncovered a vulnerability in ActiveWorkflow, we suggest
+that you get in touch via the e-mail address
+[[email protected]](mailto:[email protected]). In the message, try
+to provide a description of the issue and ideally a way of reproducing it.
+Please note that this should be used only for undisclosed vulnerabilities.