If you want to add to the documentation, it is possible to update the repository with any tool you like. However, we strongly advise to use the GitBook editor. A lot of cumbersome work is automated for you. Here are the steps to get on your way:
Create an account at Gitbook.
Once you have an account get in touch with Han Joosten so he can grant you access to the book(s) you want to contribute to.
You will automatically find the books at your Gitbook dashboard, where you will find a button to start editing the book.
The editor explains itself. At GitBook, they eat their own dogfood to write GitBook Documentation .
Recently, Gitbook also has a desktop client. It can be found here. When you install this desktop client, GitBook will make a local clone of the book. You will find it at
~home/Gitbook. As far as I know, this is not configurable, so you'll just have to deal with it. You can synchronize from within gitbook. However, if it gets a little bit complicated, I prefer sourcetree to give the correct git commands.