Create SECURITY.md (#1685)

Co-authored-by: Jeffrey Cafferata <jeffrey@jcid.nl>

* Update SECURITY.md

SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+DNSControl is a command-line tool and therefore has a different (limited) attack surface as compared to a web app or other system.
+
+## Supported Versions
+
+Only the most recent release is supported with security updates.
+
+When a major version is incremented, we'll support the previous major version for 6 months. For example, when v4.0 is released, we will support the most recent v3.x release for 6 months.
+
+## Reporting a Vulnerability
+
+To report a vulnerability please [create a new Github "issue"](https://github.com/StackExchange/dnscontrol/issues/new/choose).
+
+We will respond in a best-effort manner, usually within 1 week. We will communciate via the Github issue unless we need to communicate privately, in which case we'll arrange a way to communicate directly.
+