Add security policy document

SECURITY.md

@@ -0,0 +1,18 @@
+# Security Policy
+
+## Supported Versions
+
+Use this section to tell people about which versions of your project are
+currently being supported with security updates.
+
+| Version | Supported          | Remark                                |
+| ------- | ------------------ | ------                                |
+| 3.0.x   | :white_check_mark: | Current development release           |
+| 2.1.x   | :white_check_mark: | Stable release - security fixes only  |
+| 2.0.x   | :x:                | Previous stable release - unsupported |
+| 1.x     | :x:                | Unsupported                           |
+
+## Reporting a Vulnerability
+
+Please [open an issue](https://github.com/paulgear/ntpmon/issues/new).  At the
+moment I can see no need to create private issues for security flaws.