How do I modify robots.txt in Plone?

I've got a Plone site that I administer and I'd like to add some pages to the Disallow of a robots.txt.

It appears that Plone automatically generates a robots.txt file. I can't find any way to modify that. I've also tried adding a 'robots.txt' file to the root of the app, but it says that "robots.txt is reserved"

Does anyone know how to modify this with Plone (v3)?

1. Create a robots.txt on your desktop
2. Go in the xmi of the plone site -> add a new file -> call it robot.txt
3. Choose it from the desktop

source: http://plone.org/support/forums/general#nabble-td329779