I drew it into the Top Overlay Layer. Hope thats not a problem.
It's not really, they'll figure it out, that's their job.
It's just that if it's a thick line then you'll likely get some silkscreen around the edge of your board and that can look messy.
They use the center of the line as the marker, so some people do that and complain why their silkscreen board outline is "chopped off".
Using a thin 1thou outline shows that it's obviously for mechanical detail, but I often use 5 thou on the mechanical layer so it shows on screen better.
Having separate layers is much nice, as it allows you to turn the layer off and on at will, and then if you want to do a silkscreen board outline, it's easy to place the silkscreen line just where you want it inside the board cutout.
It is common to use a keepout layer for a board outline too, so then you can place copper pours and do DRC checks etc easily.
But really, the mechanical layers are there for this specific reason, use them.
Dave.