See JabRef's contribution guide for a howto to contribute code.
In case you aim for contributing user documentation, head to the JabRef user documetnation repository.
In case you use some gitbook special features and you want to test them, checkout JabRef's code locally, and execute following steps:
npm install -g gitbook
Then, you can see a near-to-reality rendering of the development documentation at http://localhost:4000.