Add markdown styleguide.

CONTRIBUTING.md

@@ -107,9 +107,10 @@ For examples of feedback on merge requests please look at already [closed merge
 
 ## Style guides
 1. [Ruby](https://github.com/bbatsov/ruby-style-guide)
-2. [Rails](https://github.com/bbatsov/rails-style-guide)
-3. [Formatting](https://github.com/thoughtbot/guides/tree/master/style#formatting)
-4. [Naming](https://github.com/thoughtbot/guides/tree/master/style#naming) 
-8. [Testing](https://github.com/thoughtbot/guides/tree/master/style#testing)
-7. [CoffeeScript](https://github.com/thoughtbot/guides/tree/master/style#coffeescript)
-9. [Shell commands](doc/development/shell_commands.md)
+1. [Rails](https://github.com/bbatsov/rails-style-guide)
+1. [Formatting](https://github.com/thoughtbot/guides/tree/master/style#formatting)
+1. [Naming](https://github.com/thoughtbot/guides/tree/master/style#naming) 
+1. [Testing](https://github.com/thoughtbot/guides/tree/master/style#testing)
+1. [CoffeeScript](https://github.com/thoughtbot/guides/tree/master/style#coffeescript)
+1. [Shell commands](doc/development/shell_commands.md)
+1. [Markdown](http://www.cirosantilli.com/markdown-styleguide)