bookdown::gitbook:
  css: style.css
  config:
    toc:
      before: |
        <li><a href="./">APSIM Wheat Doc</a></li>
      after: |
        <li><a href="https://github.com/byzheng/apsim-classic-wheat-doc" target="blank">Github</a></li>
    edit: https://github.com/byzheng/apsim-classic-wheat-doc/edit/master/%s