explicit note on CheckBoxFieldWidget import.

plone/app/z3cform/templates/checkbox_input.pt

@@ -18,7 +18,16 @@
            we cannot autoset the attributes here with "view/attributes",
            because the "id" would get overwritten with the field_id
            instead of the correct sequence_item_id -->
-      <input class="${view/attributes/class}"
+      <!--! NOTE #2:
+           Since most people import the "CheckBoxFieldWidget"
+           directly from z3c.form.browser.checkbox our adapter
+           which sets the Bootstrap "form-check-input" css class
+           isn't invoked. For this reason we manually add the
+           class here in the template.
+           When all imports are refactored to
+           "plone.app.z3cform.widgets.checkbox.CheckBoxFieldWidget"
+           this can be cleaned up. -->
+      <input class="${view/attributes/class} form-check-input"
              id="${item/id}"
              checked="${python:'checked' if item['checked'] else None}"
              name="${item/name}"