Add UI guide to GitLab development help

doc/development/README.md

@@ -5,3 +5,4 @@
 - [Rake tasks](rake_tasks.md) for development
 - [CI setup](ci_setup.md) for testing GitLab
 - [Sidekiq debugging](sidekiq_debugging.md)
+- [UI guide](ui_guide.md) for building GitLab with existing css styles and elements

doc/development/ui_guide.md

+# UI Guide for building GitLab 
+
+## Best practices for creating new pages in GitLab
+
+TODO: write some best practices when develop GitLab features.
+
+## GitLab UI development kit
+
+We created a page inside GitLab where you can check commonly used html and css elements.
+
+When you run GitLab instance locally - just visit http://localhost:3000/help/ui page to see UI examples 
+you can use during GitLab development.