docs: add fragment note (#385)

To add the React Fragment requirement note the Preact 'Render Optimiztion' content was copied with changes to code example and added note.  This code was tested and working the correctly with React.

https://codesandbox.io/s/preact-signals-in-react-test-mzfv8x\?file\=/src/LogCount.tsx

packages/react/README.md

@@ -66,6 +66,36 @@ function Counter() {
 }
 ```
 
+### Rendering optimizations
+
+The React adapter ships with several optimizations it can apply out of the box to skip virtual-dom rendering entirely. If you pass a signal directly into JSX, it will bind directly to the DOM `Text` node that is created and update that whenever the signal changes.
+
+```js
+import { signal } from "@preact/signals-react";
+
+const count = signal(0);
+
+// Unoptimized: Will trigger the surrounding
+// component to re-render
+function Counter() {
+	return <p>Value: {count.value}</p>;
+}
+
+// Optimized: Will update the text node directly
+function Counter() {
+	return (
+		<p>
+			<>Value: {count}</>
+		</p>
+	);
+}
+```
+
+To opt into this optimization, simply pass the signal directly instead of accessing the `.value` property.
+
+> **Note**
+> The content is wrapped in a React Fragment due to React 18's newer, more strict children types.
+
 ## License
 
 `MIT`, see the [LICENSE](../../LICENSE) file.