When you first create a new repository, a wiki attached to this project is also created. It is enabled by default and everyone can add new content or modify existing pages. If you want to change this behavior, you can refer to Chapter 6, Exploring the User and Repository Settings, which shows you how to accomplish this.
Using the wiki
Why wikis are a nice place to document your project
Documentation is not to be taken lightly. To paraphrase a famous quote, with great projects comes great documentation.
Although there are many tools that convert markup files, such as Markdown to HTML, you may not want to use an external page to host your documentation. Enter GitHub wiki.