Merge branch 'dz-improve-ui-guide' into 'master'

Write some thoughts to the UI guide

cc @skyruler 

See merge request !4290

doc/development/ui_guide.md

@@ -6,3 +6,31 @@ We created a page inside GitLab where you can check commonly used html and css e
 
 When you run GitLab instance locally - just visit http://localhost:3000/help/ui page to see UI examples 
 you can use during GitLab development.
+
+## Design repository
+
+All design files are stored in the [gitlab-design](https://gitlab.com/gitlab-org/gitlab-design) 
+repository and maintained by GitLab UX designers. 
+
+## Navigation
+
+GitLab's layout contains 2 sections: the left sidebar and the content. The left sidebar contains a static navigation menu. 
+This menu will be visible regardless of what page you visit. The left sidebar also contains the GitLab logo 
+and the current user's profile picture. The content section contains a header and the content itself.  
+The header describes the current GitLab page and what navigation is 
+available to user in this area. Depending on the area (project, group, profile setting) the header name and navigation may change. For example when user visits one of the 
+project pages the header will contain a project name and navigation for that project. When the user visits a group page it will contain a group name and navigation related to this group.
+
+### Adding new tab to header navigation
+
+We try to keep the amount of tabs in the header navigation between 5 and 10 so that it fits on a typical laptop screen. We also try not to confuse the user with too many options. Ideally each 
+tab should represent separate functionality. Everything related to the issue 
+tracker should be under the 'Issues' tab while everything related to the wiki should 
+be under 'Wiki' tab and so on and so forth.
+
+## Mobile screen size 
+
+We want GitLab to work well on small mobile screens as well. Size limitations make it is impossible to fit everything on a mobile screen. In this case it is OK to hide 
+part of the UI for smaller resolutions in favor of a better user experience. 
+However core functionality like browsing files, creating issues, writing comments, should
+be available on all resolutions.
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