If you want to make a contribution, simply commit your new markdown to the master branch and we'll take it from there. The instructions below are only for setting up your own, full development environment and are uneccessary for most people.
You're going to need NodeJS, Gitbook, and one of its plugins. Once you have NodeJS and npm, run npm install gitbook gitbook-plugin-ga -g. After that, you should be able to checkout the trailofbits/ctf repository and run gitbook serve for a local webserver. If you want to get fancy, there is a GUI editor available in GitbookIO/editor.
The CTF Field Guide is built on GitHub Pages and, therefore, there are two branches: master, for the raw markdown, and gh-pages, for the compiled website. Here's how I manage both of them on my own system.
First, clone the master branch into a folder like normal.
dan:dan $ cd ~/github
dan:github $ mkdir ctf
dan:github $ cd ctf
dan:github $ git clone https://github.com/trailofbits/ctf .
Next, clone the gh-pages branch into another folder inside the same parent. In this directory, switch to the gh-pages branch and delete the master branch.
dan:dan $ cd ~/github
dan:github $ mkdir ctf-pages
dan:github $ cd ctf-pages
dan:ctf-pages $ git clone https://github.com/trailofbits/ctf .
dan:ctf-pages $ git checkout origin/gh-pages -b gh-pages
dan:ctf-pages $ git branch -d master
dan:ctf-pages $ git branch
* gh-pages
Now you have one folder to track each branch, master and gh-pages.
When you're ready to update the website, commit your new and updated markdown to the master branch as normal. Then run the to_gh.sh script to have gitbook build the book and copy it into the ctf-pages directory. Move over to the ctf-pages directory and run git_yolo.sh there (I'm great at git, I know). This will commit the new, compiled website to the gh-pages branch and update the website.