code: public final FacetOptions addFacetOnField(Field field) {
  		Assert.notNull(field, "Cannot facet on null field.");
  		Assert.hasText(field.getName(), "Cannot facet on field with null/empty fieldname.");
  
  		this.facetOnFields.add(field);
  		return this;
  	}
docs: Append additional field for faceting
  
  @param field
  @return