Add references to the rouge gem library

[ci skip]

doc/markdown/markdown.md

@@ -88,6 +88,9 @@ GFM will autolink almost any URL you copy and paste into your text.
 
 ## Code and Syntax Highlighting
 
+_GitLab uses the [rouge ruby library][rouge] for syntax highlighting. For a
+list of supported languages visit the rouge website._
+
 Blocks of code are either fenced by lines with three back-ticks <code>```</code>, or are indented with four spaces. Only the fenced code blocks support syntax highlighting.
 
 ```no-highlight
@@ -585,3 +588,5 @@ By including colons in the header row, you can align the text within that column
 - This document leveraged heavily from the [Markdown-Cheatsheet](https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet).
 - The [Markdown Syntax Guide](https://daringfireball.net/projects/markdown/syntax) at Daring Fireball is an excellent resource for a detailed explanation of standard markdown.
 - [Dillinger.io](http://dillinger.io) is a handy tool for testing standard markdown.
+
+[rouge]: http://rouge.jneen.net/ "Rouge website"

doc/profile/preferences.md

@@ -12,6 +12,9 @@ The default is **Charcoal**.
 
 ## Syntax highlighting theme
 
+_GitLab uses the [rouge ruby library][rouge] for syntax highlighting. For a
+list of supported languages visit the rouge website._
+
 Changing this setting allows the user to customize the theme used when viewing
 syntax highlighted code on the site.
 
@@ -36,3 +39,5 @@ The default is **Your Projects**.
 It allows user to choose what content he or she want to see on project page.
 
 The default is **Readme**.
+
+[rouge]: http://rouge.jneen.net/ "Rouge website"