Hi Bryan,
I think you're probably thinking about attaching Release Notes/Commercialization documents, and the like, to the Release object. Not a bad idea, but that's not possible right now. Releases used in the context of PSIs, in my opinion, should have this ability.
In the meantime, as David suggests, adding a link to your document (whether it's in SharePoint/Wiki/Confluence etc.) could be a decent workaround.
Please submit an idea at this site: ideas.rallydev.com (but search for it first in case there's already an idea similar to this).
Sesh