The documentation template Landon pointed out is a great place to start. Of course, you won't have to bother with any of the code examples inside of that. Things I would consider adding to your PSD document are:
- A list of PSD flies included (like contact page, blog page, portfolio, etc.)
- Any credit or sources you need to provide (like fonts).
- A basic primer on the layers you've used might be helpful.
That's what I would do, at least.