Add changelog entry for #4562

CHANGELOG.md

@@ -16,6 +16,16 @@ These additional styles are not included if you use `govuk-font-tabular-numbers`
 
 This change was introduced in [pull request #4307: Refactor tabular number activation into their own mixin](https://github.com/alphagov/govuk-frontend/pull/4307)
 
+### Access our breakpoint values as CSS custom properties
+
+We have added the following CSS custom properties to GOV.UK Frontend:
+
+- `--govuk-frontend-breakpoint-mobile: 20rem;`
+- `--govuk-frontend-breakpoint-tablet: 40.0625rem;`
+- `--govuk-frontend-breakpoint-desktop: 48.0625rem;`
+
+You can access these in your custom CSS and JavaScript to get the GOV.UK Frontend media query breakpoint values.
+
 ### Recommended changes
 
 #### Replace instances of `govuk-typography-responsive` with `govuk-font-size`