docs: Add SECURITY.md

I've enabled the "Private vulnerability reporting" at https://github.com/gruntjs/grunt-contrib-connect/security.

This matches gruntjs/grunt, per
gruntjs/grunt#1770.

Author: Krinkle

SECURITY.md

@@ -0,0 +1,5 @@
+## Reporting a vulnerability
+
+If you discover a security vulnerability within grunt, please submit a report via the Github's Private Vulnerability Reporting feature.
+
+All security vulnerabilities will be promptly addressed.