Finish CI guide.

site/docs/continuous-integration.md

@@ -14,11 +14,57 @@ private repositories.
 [0]: https://travis-ci.org/
 [1]: https://github.com/
 
+## Enabling Travis and GitHub
+
+Enabling Travis builds for your GitHub repository is pretty simple:
+
+1. Go to your profile on travis-ci.org: https://travis-ci.org/profile/username
+2. Find the repository for which you're interested in enabling builds.
+3. Click the slider on the right so it says "ON" and is a dark grey.
+4. Optionally configure the build by clicking on the wrench icon. Further
+   configuration happens in you `.travis.yml` file. More details on that
+   below.
+
 ## The Test Script
 
-When testing static sites, there is no better tool than [html-proofer][].
+The simplest test script simply runs `jekyll build` and ensures that Jekyll
+doesn't fail to build the site. It doesn't check the resulting site, but it
+does ensure things are built properly.
 
-## Enabling Travis and GitHub
+When testing Jekyll output, there is no better tool than [html-proofer][2].
+This tool checks your resulting site to ensure all links and images exist.
+Utilize it either with the convenient `html-proof` command-line executable,
+or write a Ruby script which utilizes the gem.
+
+### The `html-proof` Executable
+
+```bash
+#!/usr/bin/env bash
+
+jekyll build
+html-proof ./_site
+```
+
+Some options can be specified via command-line switches. Check out the
+`html-proofer` README for more information about these switches, or run
+`html-proof --help` locally.
+
+### The `html/proofer` Library
+
+You can also invoke `html-proofer` in Ruby scripts (e.g. in a Rakefile):
+
+```ruby
+#!/usr/bin/env ruby
+
+require 'html/proofer'
+HTML::Proofer.new("./_site").run
+```
+
+Options are given as a second argument to `.new`, and are encoded in a
+symbol-keyed Ruby Hash. More information about the configuration options,
+check out `html-proofer`'s README file.
+
+[2]: https://github.com/gjtorikian/html-proofer
 
 ## Configuring Your Travis Builds
 
@@ -26,3 +72,105 @@ This file is used to configure your Travis builds. Because Jekyll is built
 with Ruby and requires RubyGems to install, we use the Ruby language build
 environment. Below is a sample `.travis.yml` file, and what follows that is
 an explanation of each line.
+
+```yaml
+language: ruby
+rvm:
+- 2.1
+script: ./script/cibuild
+
+# branch whitelist
+branches:
+  only:
+  - gh-pages     # test the gh-pages branch
+  - /pages-(.*)/ # test every branch which starts with "pages-"
+
+env:
+  global:
+  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
+```
+
+Ok, now for an explanation of each line:
+
+{% highlight yaml %}
+language: ruby
+{% endhighlight %}
+
+This line tells Travis to use a Ruby build container. It gives your script
+access to Bundler, RubyGems, and and Ruby runtime.
+
+{% highlight yaml %}
+rvm:
+- 2.1
+{% endhighlight %}
+
+RVM is a popular Ruby Version Manager (like rbenv, chruby, etc). This
+directive tells Travis the Ruby version to use when running your test
+script.
+
+{% highlight yaml %}
+script: ./script/cibuild
+{% endhighlight %}
+
+Travis allows you to run any arbitrary shell script to test your site. One
+convention is to put all scripts for your project in the `script`
+directory, and to call your test script `cibuild`. This line is completely
+customizable. If your script won't change much, you can write your test
+incantation here directly:
+
+{% highlight yaml %}
+script: jekyll build && html-proof ./_site
+{% endhighlight %}
+
+The `script` directive can be absolutely any valid shell command.
+
+{% highlight yaml %}
+# branch whitelist
+branches:
+  only:
+  - gh-pages     # test the gh-pages branch
+  - /pages-(.*)/ # test every branch which starts with "pages-"
+{% endhighlight %}
+
+You want to ensure the Travis builds for your site are being run only on
+the branch or branches which contain your site. One means of ensuring this
+isolation is including a branch whitelist in your Travis configuration
+file. By specifying the `gh-pages` branch, you will ensure the associated
+test script (discussed above) is only executed on site branches. If you use
+a pull request flow for proposing changes, you may wish to enforce a
+convention for your builds such that all branches containing edits are
+prefixed, exemplified above with the `/pages-(.*)/` regular expression.
+
+The `branches` directive is completely optional.
+
+{% highlight yaml %}
+env:
+  global:
+  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
+{% endhighlight %}
+
+Using `html-proofer`? You'll want this environment variable. Nokogiri, used
+to parse HTML files in your compiled site, comes bundled with libraries
+which it must compile each time it is installed. Luckily, you can
+dramatically increase the install time of Nokogiri by setting the
+environment variable `NOKOGIRI_USE_SYSTEM_LIBRARIES` to `true`.
+
+## Gotchas
+
+### Exclude `vendor`
+
+Travis bundles all gems in the `vendor` directory on its build servers,
+which Jekyll will mistakenly read and explode on. To avoid this, exclude
+`vendor` in your `_config.yml`:
+
+{% highlight yaml %}
+exclude: [vendor]
+{% endhighlight %}
+
+### Questions?
+
+This entire guide is open-source. Go ahead and [edit it][3] if you have a
+fix or [ask for help][4] if you run into trouble and need some help.
+
+[3]: https://github.com/jekyll/jekyll/edit/master/site/docs/continuous-integration.md
+[4]: https://github.com/jekyll/jekyll-help#how-do-i-ask-a-question