Added another breaking change to the migration guide

If you have used any of the shadow-buttons mixins they have been renamed

CHANGELOG.md

@@ -99,11 +99,12 @@ If you want to keep using it and don't plan on upgrading to `ffe-buttons@2.1.0`
 }
 ```
 
+If you have used any of the `.ffe-shadow-buttons-whatever()` mixins they are now `.ffe-buttons-shadow-whatever()`.
+
 #### "Gotchas"
 
 * `ffe-tab-button` is in `ffe-tabs`, not `ffe-buttons`
 
-
 ## v.7.0.1
 * Minor fix: make button texts of loading buttons unselectable, the text is only of interest for
   screen readers and ought to remain invisible under all circumstances.