Merge pull request #1749 from alphagov/document-class-name-structure

Add note about override naming for media queries

src/govuk/README.md

@@ -70,6 +70,6 @@ and override all other layers. Rules in this layer will generally be marked as
 Class names within this layer use an additional prefix `-!-`, for example
 `.govuk-!-font-width-regular`.
 
-
+If a class only overrides when a condition is met, the condition will be at the end of the class name. For example `.govuk-!-display-none-print` will only override with `display: none` in a print layout.
 
 [ITCSS]: (http://www.creativebloq.com/web-design/manage-large-css-projects-itcss-101517528)