change setState to use a function

Also added note to readme

step1-04/demo/README.md

@@ -168,7 +168,7 @@ Our next step is to wire up the button to increment the `counter` in our compone
 
 ```jsx
 _onButtonCLick = () => {
-  this.setState({ counter: this.state.counter + 1 });
+  this.setState(prevState => ({ counter: prevState.counter + 1 }));
 };
 ```
 
@@ -176,6 +176,8 @@ This function will update our component state, incrementing our counter value by
 
 > Note that this isn't exactly a method, but a property that is equal to a [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions). This works just as well as `onButtonClick() { }`, but doesn't require extra binding up in the constructor.
 
+> Also note that `setState()` may update the state [asynchronously](https://reactjs.org/docs/state-and-lifecycle.html#state-updates-may-be-asynchronous). If the next state depends on the previous state or props, then we should use the second form of `setState()` which takes a function that receives the previous state as the first argument and the props as the second.
+
 Now that we have a function to increment out count, all that we have left is to connect it to our button.
 
 ```jsx

step1-04/final/src/components/Counter.tsx

@@ -14,12 +14,12 @@ export class Counter extends React.Component<any, any> {
     return (
       <div>
         {text}: {counter}
-        <Button onClick={this._onButtonCLick}>Click</Button>
+        <Button onClick={this._onButtonClick}>Click</Button>
       </div>
     );
   }
 
-  _onButtonCLick = () => {
-    this.setState({ counter: this.state.counter + 1 });
+  _onButtonClick = () => {
+    this.setState((state) => ({ counter: state.counter + 1 }));
   };
 }