Merge branch 'master' into 'master'

Added documentation styleguide

Hi. I added a documentation styleguide and linked it to https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides

See merge request !709

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](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).

doc_styleguide.md

+# Documentation styleguide
+
+This styleguide recommends best practices to improve documentation and to keep it organized and easy to find. 
+
+## Text
+
+* Make sure that the documentation is added in the correct directory and that there's a link to it somewhere useful.
+
+* Add only one H1 or title in each document, by adding '#' at the begining of it (when using markdown). For subtitles, use '##', '###' and so on.
+
+* Do not duplicate information.
+
+* Be brief and clear.
+
+
+## When adding images to a document
+
+* Create a directory to store the images with the specific name of the document where the images belong. It could be in the same directory where the .md document that you're working on is located.
+
+* Images should have a specific, non-generic name that will differentiate them.
+
+* Keep all file names in lower case.
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