Add useState example (#128)

resolve #120

Author: sosukesuzuki

Diff for: README.md

@@ -770,6 +770,31 @@ export default () => (
 
 > https://reactjs.org/docs/hooks-intro.html
 
+#### - useState
+
+> https://reactjs.org/docs/hooks-reference.html#usestate
+
+```tsx
+import * as React from 'react';
+
+type Props = { initialCount: number };
+
+export default function Counter({initialCount}: Props) {
+  const [count, setCount] = React.useState(initialCount);
+  return (
+    <>
+      Count: {count}
+      <button onClick={() => setCount(initialCount)}>Reset</button>
+      <button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
+      <button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
+    </>
+  );
+}
+
+```
+
+[⇧ back to top](#table-of-contents)
+
 #### - useReducer
 Hook for state management like Redux in a function component.
 

Diff for: README_SOURCE.md

@@ -313,6 +313,14 @@ const mapDispatchToProps = (dispatch: Dispatch<ActionType>) => ({
 
 > https://reactjs.org/docs/hooks-intro.html
 
+#### - useState
+
+> https://reactjs.org/docs/hooks-reference.html#usestate
+
+::codeblock='playground/src/hooks/use-state.tsx'::
+
+[⇧ back to top](#table-of-contents)
+
 #### - useReducer
 Hook for state management like Redux in a function component.
 

Diff for: playground/src/hooks/use-state.tsx

@@ -0,0 +1,15 @@
+import * as React from 'react';
+
+type Props = { initialCount: number };
+
+export default function Counter({initialCount}: Props) {
+  const [count, setCount] = React.useState(initialCount);
+  return (
+    <>
+      Count: {count}
+      <button onClick={() => setCount(initialCount)}>Reset</button>
+      <button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
+      <button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
+    </>
+  );
+}