on any worldspawn:
Key: message
Value: anything you want
rrrriiiiiight.... I had already forgotten about that one

Well, im a noob, seriously, but howto put that worldspawn thing D=
"worldspawn" is the collection of all your 'normal' brushes,
anything that's not a special entity, like doors or elevators.
Select a normal brush and press the N key. You can add options there.