You might have a
doc folder in your project, full of documentation you maintain as you work on it.
You might also have a wiki on github with even more documentation, that isn't in the source (or documentation in the source that isn't in the wiki).
You can keep them in sync with Git, and Github is smart enough to know:
# change 'docs' at the end to whatever... or just omit it git submodule add email@example.com:USER/REPO.wiki.git docs
Copy your other docs into the submodule repo, if you want, and then commit and push. Wiki updated!
You can't access the wiki repo directly, like you can your own. However, when you try to navigate to it in the source tree on Github, it'll take you straight to the wiki.
The end result is documentation that sits inside your working copy for offline reference, that can easily be synced with the wiki, and vice versa.
It also means potential users always see an up-to-date wiki, without having to clone the repo to get at the real documentation.