Remove old security diclaimer from contributing.md file

Author: daleksan

CONTRIBUTING.md

@@ -47,7 +47,7 @@ Once you've finished making your changes push them to your fork and send the PR
 
 ## Reporting a security issue
 
-Please mail to [email protected] directly for security issue
+Please refer to [security.md](security.md) file for details.
 
 ## Public issue tracking