Every repository on GitHub can have a wiki enabled for it. It’s not the most complete wiki platform, but it does make it very easy to host documentation without any fuss.

As an example, take a look at the Resque repository, which has a very simple wiki containing a few of the things that you usually don’t add to the main repository. In this case there are a few links to presentations about the project, and lists of links to similar projects.

The key difference between repositories and wikis are that wikis are open by default. Anybody can make changes to the wiki without having to clone or create a pull request. As the wiki owner, you will not receive any notifications to changes that others make to your wiki.

You can enable your repository wiki using the GitHub web interface, in the Settings tab of your repository (in the list of buttons on the right.)

Bonus

A GitHub wiki is also a Git repository. So you can have a local copy of everything, and even make wiki changes locally before pushing them to GitHub. To clone it, add a .wiki to the url, like so:

$ git clone https://github.com/OpenTechSchool-Dortmund/underground.wiki.git