Add note about failing contrast ratio

We’ve decided not to remedy this for now, as the `disabled` state
and other attributes of the publish button suffice, but in the future
it would be good to find a better contrast solution here.

src/styles/mixins/forms.scss

@@ -80,6 +80,7 @@
   padding-right: 12px;
 
   &:disabled {
+    // NB: This doesn't allow for enough contrast ratio to meet WCAG AA requirements
     color: var.$grey-semi;
   }