It is very easy to add text to the login window in OS X, This can be useful to display an “If Found” message in the unfortunate event that you loose your precious OS X device. By adding a message, hopefully, if your device is found, the finder will have a way of contacting you and reunite you with your device!
Open System Preferences.
Under the Personal tab click on Security & Privacy
Select the General tab then put a tick in the 2nd box “Show a message when the screen is locked”
Edit the message using the “Set Lock Message” button.
This message now displays near the bottom of the Login window or the lock screen. You can make multiple paragraphs by pressing Option-Return to go to a new line.