enable webpack hmr

packages/react-scripts/template/src/index.tsx

@@ -4,8 +4,25 @@ import App from './App';
 import './index.css';
 import registerServiceWorker from './registerServiceWorker';
 
-ReactDOM.render(
-  <App />,
-  document.getElementById('root') as HTMLElement
-);
+function registerHMR() {
+  type ModuleHMR = typeof module & {
+    hot?: {
+      accept(dependencies: string | string[], callback: (updatedDependencies: any[]) => void): void
+    }
+  };
+
+  if ((module as ModuleHMR).hot) {
+    (module as ModuleHMR).hot!.accept('./App', render);
+  }
+}
+
+function render() {
+  ReactDOM.render(
+    <App />,
+    document.getElementById('root') as HTMLElement
+  );
+}
+
 registerServiceWorker();
+registerHMR();
+render();