This is very handy to notify users about scheduled maintenances.
All you have to do is go into transaction SE61, type in ZLOGIN_SCREEN_INFO at the Data element field, select General text at the Document Class (and also the language if not English) and click on Create button (image below).
Now write the message.
Tip: You can put icons in the message using @XX@ in the message, where XX is the number of the icon (which can be found at SA38, by running the program SHOWICON):
When the user start the SAP R/3, he will see the message on the login screen: