The login screen of OS X by default only shows the computer name. It is however possible to add a message or warning to the login screen. Use following command to do this.
sudo defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "Your Message"
Replace “My message” with the text that you would like to be displayed.
Revert to defaults setting:
sudo defaults delete /Library/Preferences/com.apple.loginwindow LoginwindowText