I do not keep a note for each layer of my layout (each button, each ribbon, each paper) since I tend to only use one kit for each layout. But to keep track of what kit(s) or templates I use on a layout I put it into the file info for that layout. In Photoshop (both CS and PSE) go File>File Info In the comments/description box, type in your credits and click save/OK.
I like this method because it stores the information with the file, no matter how you save it (TIFF, PSD, JPEG). If you want the info, you do not have to open the layout, just right click on it and go to properties and you can copy it from there to paste into the gallery.