Added .mkdn as a Markdown extension

I have already submitted a corresponding change to Linguist (see #1215)

README.md

@@ -10,7 +10,7 @@ Markups
 The following markups are supported.  The dependencies listed are required if
 you wish to run the library. You can also run `script/bootstrap` to fetch them all.
 
-* [.markdown, .mdown, .md](http://daringfireball.net/projects/markdown/) -- `gem install redcarpet` (https://github.com/vmg/redcarpet)
+* [.markdown, .mdown, .mkdn, .md](http://daringfireball.net/projects/markdown/) -- `gem install redcarpet` (https://github.com/vmg/redcarpet)
 * [.textile](http://www.textism.com/tools/textile/) -- `gem install RedCloth`
 * [.rdoc](http://rdoc.sourceforge.net/) -- `gem install rdoc -v 3.6.1`
 * [.org](http://orgmode.org/) -- `gem install org-ruby`