Include checkbox in README.md

This includes the missing checkbox component for the sorting demo.

README.md

@@ -93,6 +93,10 @@ Here the `ListView` will only show elements that contains the content of the `Te
             RoleSorter { roleName: "lastName" }
         ]
     }
+
+    CheckBox {
+        id:onlyShowFavoritesCheckbox
+    }
 ```
 This will show in the corresponding `ListView` only the elements where the `firstName` or the `lastName` match the text entered in the `textField`, and if the `onlyShowFavoritesCheckbox` is checked it will aditionnally filter the elements where `favorite` is `true`.
 The favorited elements will be shown first and all the elements are sorted by `firstName` and then `lastName`.