add warning on app.ini

Author: lunny

custom/conf/app.ini.sample

@@ -32,6 +32,7 @@ PREFERRED_LICENSES = Apache License 2.0,MIT License
 ; Disable the ability to interact with repositories using the HTTP protocol
 DISABLE_HTTP_GIT = false
 ; Value for Access-Control-Allow-Origin header, default is not to present
+; WARNING: This maybe harmful to you website if you do not give it a right value.
 ACCESS_CONTROL_ALLOW_ORIGIN = 
 ; Force ssh:// clone url instead of scp-style uri when default SSH port is used
 USE_COMPAT_SSH_URI = false