The documentation you submit strongly influences your subjective mark on the project. In the past, the projects receiving the highest subjective marks had excellent documentation. Often, two projects would have approximately the same technical merit, but one would receive a much better mark due to differences in documentation. Here's a brief rundown of what we look for in documentation:
A discussion of the interesting technical points of the project. You should note what is interesting and what is required to implement the idea. A map of the code. We don't want manual pages, but would like pointers to where we can find the various features you have implemented. References. You should note the key technical references for your project. Acknowledgments. You should credit any code written by other people and acknowledge people who have helped you.