If you want to have map briefings available in your maps, create a normal .txt file and name it 'yourmapname'.txt (obviously you don't literally call it 'yourmapname' but replace it with whatever your map is called.)

To get the briefing to show, use the command menu ( console commmand +hud) and navigate to the briefing option, or use the briefing command +briefing.

You can use a trigger_command in your maps to make the briefing pop up.