Update upgrade guide

Author: colinrotherham

docs/upgrade-to-6.0.md

@@ -18,6 +18,10 @@ The updated [header](https://service-manual.nhs.uk/design-system/components/head
 - update NHS logo in the header to have higher contrast when focused
 - refactor CSS classes and BEM naming, use hidden attributes instead of modifier classes, use generic search element
 
+#### Smaller versions of radio buttons and checkboxes
+
+You can now use smaller versions of the [radios](https://service-manual.nhs.uk/design-system/components/radios) and [checkboxes](https://service-manual.nhs.uk/design-system/components/checkboxes) components by adding the `small` prop.
+
 ### Panel component
 
 The [panel](https://service-manual.nhs.uk/design-system/components/panel) component from NHS.UK frontend v9.3.0 has been added: