Inline docs: Add link to YAML wikipedia article

Official resources about YAML are ... - impossible to understand for a ordinary user.

content-sample/index.md

@@ -72,8 +72,8 @@ follows: <code>!\[Image Title\](&#37;base_url&#37;/assets/image.png)</code>
 
 Text files are marked up using [Markdown][]. They can also contain regular HTML.
 
-At the top of text files you can place a block comment and specify certain
-attributes of the page. For example:
+At the top of text files you can place a block comment and specify certain meta
+attributes of the page using [YAML][] (the "YAML header"). For example:
 
     ---
     Title: Welcome
@@ -285,6 +285,7 @@ setting `$config['rewrite_url'] = true;` in your `config/config.php`.
 For more help have a look at the Pico documentation at http://picocms.org/docs.
 
 [Markdown]: http://daringfireball.net/projects/markdown/syntax
+[YAML]: https://en.wikipedia.org/wiki/YAML
 [Twig]: http://twig.sensiolabs.org/documentation
 [WikiThemes]: https://github.com/picocms/Pico/wiki/Pico-Themes
 [WikiPlugins]: https://github.com/picocms/Pico/wiki/Pico-Plugins