Merge branch 'snake-case' into 'master'

Update style_guide_scss.md

See merge request gitlab-org/gitlab-ce!16651

doc/development/fe_guide/style_guide_scss.md

@@ -7,6 +7,8 @@ easy to maintain, and performant for the end-user.
 ### Naming
+Filenames should use `snake_case`.
+
 CSS classes should use the `lowercase-hyphenated` format rather than
 `snake_case` or `camelCase`.