docs: update usage

Author: barelyhuman

README.md

@@ -4,14 +4,54 @@
 
 [![Test](https://github.com/barelyhuman/babel-transform-mutable-react-state/actions/workflows/test.yml/badge.svg)](https://github.com/barelyhuman/babel-transform-mutable-react-state/actions/workflows/test.yml)
 
+**UNSTABLE**
+**The plugin is still under development so isn't recommended for production**
+
 ## Install
 
 TBD
 
 ## Usage
 
-**UNSTABLE**
-The plugin is still under development so isn't recommended for production
+You write state with a prefix `$` and that's converted to `useState` accordingly.
+
+```jsx
+import * as React from 'react'
+
+function Component() {
+	let $a = 1
+
+	const onPress = () => {
+		$a += 1
+	}
+
+	return (
+		<div>
+			<p>{$a}</p>
+			<button onClick={onPress}>Press</button>
+		</div>
+	)
+}
+```
+
+```jsx
+import * as React from 'react'
+
+function Component() {
+	const [a, setA] = React.useState(1)
+
+	const onPress = () => {
+		setA(a + 1)
+	}
+
+	return (
+		<div>
+			<p>{a}</p>
+			<button onClick={onPress}>Press</button>
+		</div>
+	)
+}
+```
 
 ## License