message: export `RuleGroup` just like `Rule`
diff: @@ -3,6 +3,7 @@ export { QueryBuilder };
  export * from './defaults';
  export * from './utils';
  export { Rule } from './Rule';
  +import { RuleGroup } from './RuleGroup';
  export { InlineCombinator } from './InlineCombinator';
  export * from './controls/';
  export * from './types';
  