Adds documentation about the utility classes

doc/development/fe_guide/style_guide_scss.md

@@ -5,6 +5,15 @@ easy to maintain, and performant for the end-user.
 
 ## Rules
 
+### Utility Classes
+As part of the effort for [cleaning up our CSS and moving our components into GitLab-UI](https://gitlab.com/groups/gitlab-org/-/epics/950)
+led by the [GitLab UI WG](https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/20623) we prefer the use of utility classes over adding new CSS. However, complex CSS can be addressed by adding component classes.
+
+We have a few internal utility classes in [`common.scss`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/common.scss)
+and we use [Bootstrap's Utility Classes](https://getbootstrap.com/docs/4.3/utilities/)
+
+New utility classes should be added to [`common.scss`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/framework/common.scss).
+
 ### Naming
 
 Filenames should use `snake_case`.