Update ES6 class documentation with binding perf

Adding a note in the ES6 class documentation about function binding.  Recommending that you bind your handlers in the constructor so that they are referentially the same function every time render is invoked (helps with child components that might potentially call shouldComponentUpdate)

docs/05-reusable-components.md

@@ -205,13 +205,14 @@ export class Counter extends React.Component {
   constructor(props) {
     super(props);
     this.state = {count: props.initialCount};
+    this.tick = this.tick.bind(this);
   }
   tick() {
     this.setState({count: this.state.count + 1});
   }
   render() {
     return (
-      <div onClick={this.tick.bind(this)}>
+      <div onClick={this.tick}>
         Clicks: {this.state.count}
       </div>
     );
@@ -223,7 +224,34 @@ Counter.defaultProps = { initialCount: 0 };
 
 ### No Autobinding
 
-Methods follow the same semantics as regular ES6 classes, meaning that they don't automatically bind `this` to the instance. You'll have to explicitly use `.bind(this)` or [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) `=>`.
+Methods follow the same semantics as regular ES6 classes, meaning that they don't automatically bind `this` to the instance. You'll have to explicitly use `.bind(this)` or [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) `=>`:
+
+```javascript
+// You can use bind() to preserve `this`
+<div onClick={this.tick.bind(this)}>
+
+// Or you can use arrow functions
+<div onClick={() => this.tick()}>
+```
+
+We recommend that you bind your event handlers in the constructor so they are only bound once for every instance:
+
+```javascript
+constructor(props) {
+  super(props);
+  this.state = {count: props.initialCount};
+  this.tick = this.tick.bind(this);
+}
+```
+
+Now you can use `this.tick` directly as it was bound once in the constructor:
+
+```javascript
+// It is already bound in the constructor
+<div onClick={this.tick}>
+```
+
+This is better for performance of your application, especially if you implement [shouldComponentUpdate()](/react/docs/component-specs.html#updating-shouldcomponentupdate) with a [shallow comparison](/react/docs/shallow-compare.html) in the child components.
 
 ### No Mixins