public static FieldUpdate createAdd(Field field, FieldValue value) {
        return new FieldUpdate(field, ValueUpdate.createAdd(value));
    }
Creates a new field update, with one encapsulated value update
specifying an addition of a value to an array or a key to a weighted set (with default weight 1).

@param field the Field to add a value to
@param value the value to add to the array, or key to add to the weighted set
@return a FieldUpdate specifying the addition
@throws IllegalArgumentException      if the runtime type of newValue does not match the type required by field
@throws UnsupportedOperationException if the field type is not array or weighted set
@see com.yahoo.document.update.ValueUpdate#createAdd(FieldValue)
document/src/main/java/com/yahoo/document/update/FieldUpdate.java