Render using GitHub Flavored Markdown

Fixes probot/probot#425, which is about the numbering for the Three Laws of Robotics list on the "Best Practices" page starting from 1, not 0.

_config.yml

@@ -13,6 +13,8 @@ exclude:
   - script
   - vendor
 
+markdown: GFM
+
 sass:
   style: :compressed
   sass_dir: _sass
@@ -44,4 +46,4 @@ plugins:
   - jekyll-readme-index
   - jemoji
 
-branch: master
+branch: master