private void addSearchFieldSet() {
        for (SDField searchField : schema.extraFieldList()) {
            schema.fieldSets().addBuiltInFieldSetItem(SEARCH_FIELDSET_NAME, searchField.getName());
        }
    }
Adds field sets for 1) fields defined inside document type 2) fields inside search but outside document

@author Vegard Havdal
config-model/src/main/java/com/yahoo/schema/processing/BuiltInFieldSets.java