Update CONTRIBUTING.md

Author: cebe

CONTRIBUTING.md

@@ -1,8 +1,8 @@
 How to contribute to Yii?
 =========================
 
-> Note that only PHP 7 compatibility issues are accepted since Yii 1.1 has reached EOL.
-  For security issues please contact maintainers privately.
+> **Note that only PHP 7 compatibility issues are accepted since Yii 1.1 has reached EOL.
+  For security issues please [contact maintainers privately](http://www.yiiframework.com/security/).**
 
 When contributing to Yii you have to follow some conventions and a simple
 workflow. It allows us to handle the requests quickly and ensure best quality