Almost always include a readme file. What's in it depends on the images. For scanned images I ususally add info about camera, and lens used (if known), film used, time/date for each picture and scanner type and settings applied during scan. Basically what I know about technical info + date that would be covered by exif info from a digicam.
For digicam pictures the above is not necessary, maybe except for date (so I don't have to look it up in the EXIF info).
For both scanned and digicam I also try to make a short comment on location, subject and/or people in the picture where that's relevant info.
It's a time consuming task and I can't say I follow up 100%, but I try. It's incredibel what you can forget in a few years.
The above is for personal use. CDs I send away is usually stripped of technical info.
