Add utility-first css section to fe docs

doc/development/fe_guide/style_guide_scss.md

@@ -29,6 +29,25 @@ New utility classes should be added to [`utilities.scss`](https://gitlab.com/git
 - `{shade}` is on of the shades listed on [colors](https://design.gitlab.com/foundations/colors/)
 - `{size}` is a number from 1-6 from our [Type scale](https://design.gitlab.com/foundations/typography)
 
+#### When should I create component classes?
+
+We recommend a "utility-first" approach. 
+
+1. Start with utility classes.
+2. If composing utility classes into a component class removes code duplication and encapsulates a clear responsibility, do it.
+
+This encourages an organic growth of component classes and prevents the creation of one-off unreusable classes. Also, the kind of classes that emerge from "utility-first" tend to be design-centered (e.g. `.button`, `.alert`, `.card`) rather than domain-centered (e.g. `.security-report-widget`, `.commit-header-icon`). 
+
+Examples of component classes that were created using "utility-first" include:
+
+- [`.circle-icon-container`](https://gitlab.com/gitlab-org/gitlab-ce/blob/579fa8b8ec7eb38d40c96521f517c9dab8c3b97a/app/assets/stylesheets/framework/icons.scss#L85)
+- [`.d-flex-center`](https://gitlab.com/gitlab-org/gitlab-ce/blob/900083d89cd6af391d26ab7922b3f64fa2839bef/app/assets/stylesheets/framework/common.scss#L425)
+
+Inspirations:
+
+- https://tailwindcss.com/docs/utility-first
+- https://tailwindcss.com/docs/extracting-components 
+
 ### Naming
 
 Filenames should use `snake_case`.