added link to documentation style guide

CONTRIBUTING.md

@@ -163,6 +163,7 @@ If you add a dependency in GitLab (such as an operating system package) please c
 1.  [Shell commands](doc/development/shell_commands.md) created by GitLab contributors to enhance security
 1.  [Markdown](http://www.cirosantilli.com/markdown-styleguide)
 1.  [Database Migrations](doc/development/migration_style_guide.md)
+1.  [Documentation styleguide](gitlab-ce/doc_styleguide.md)
 1.  Interface text should be written subjectively instead of objectively. It should be the gitlab core team addressing a person. It should be written in present time and never use past tense (has been/was). For example instead of "prohibited this user from being saved due to the following errors:" the text should be "sorry, we could not create your account because:". Also these [excellent writing guidelines](https://github.com/NARKOZ/guides#writing).
 This is also the style used by linting tools such as [RuboCop](https://github.com/bbatsov/rubocop), [PullReview](https://www.pullreview.com/) and [Hound CI](https://houndci.com).