Merge branch 'ux-guide-index' into 'master'

Fix link to index.md UX guide in development README.md

Fix wrong link to UX docs that introduced by
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7142

See merge request !7444

doc/development/README.md

@@ -14,7 +14,7 @@
   contributing to documentation.
 - [SQL Migration Style Guide](migration_style_guide.md) for creating safe SQL migrations
 - [Testing standards and style guidelines](testing.md)
-- [UX guide](ux_guide/README.md) for building GitLab with existing CSS styles and elements
+- [UX guide](ux_guide/index.md) for building GitLab with existing CSS styles and elements
 - [Frontend guidelines](frontend.md)
 - [SQL guidelines](sql.md) for working with SQL queries
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers