Selecting Files and Folders which get backed up by File History

I use File History to back up my files. I don't see a way to select anything other than the default folders. Excluding folders is straightforward. For instance, I would like to back up my public documents folder using File History. How to do that?

@Atom
To add a folder to file history, you need to include the folder in a library. To do this you right click on the folder and select the include in library option. Then just make sure you haven't excluded the library you added your non-default folder to.

I don't seem to be able to add anything to the default library. Is that the way it is supposed to be?
If I want to have something backed up to File History, I have to create a new library?