Export flow types (react-grid-layout#672)

Export flow types from entry point

Author: TrySound

index.js.flow

@@ -0,0 +1,10 @@
+// @flow
+
+import * as utils from './build/utils';
+export { default } from './build/ReactGridLayout';
+export { default as Responsive } from './build/ResponsiveReactGridLayout';
+export { default as WidthProvider } from './build/components/WidthProvider';
+
+export {
+  utils
+};