add docs on classSet add-on

Author: chenglou

docs/_config.yml

@@ -58,6 +58,8 @@ nav_docs_sections:
       title: Animation
     - id: form-input-binding-sugar
       title: Form Input Binding Sugar
+    - id: class-name-manipulation
+      title: Class Name Manipulation
   - id: examples
     title: Examples
 - title: Reference

docs/docs/09.2-form-input-binding-sugar.md

@@ -4,7 +4,7 @@ title: Form Input Binding Sugar
 layout: docs
 permalink: form-input-binding-sugar.html
 prev: animation.html
-next: examples.html
+next: class-name-manipulation.html
 ---
 
 `ReactLink` is an easy way to express two-way binding with React.

docs/docs/09.3-class-name-manipulation.md

@@ -0,0 +1,46 @@
+---
+id: class-name-manipulation
+title: Class Name Manipulation
+layout: docs
+permalink: class-name-manipulation.html
+prev: form-input-binding-sugar.html
+next: examples.html
+---
+
+`classSet()` is a neat utility for easily manipulating the DOM `class` string.
+
+Here's a common scenario and its solution without `classSet()`:
+
+```javascript
+// inside some `<Message />` React component
+render: function() {
+  var classString = 'message';
+  if (this.props.isImportant) {
+    classString += ' message-important';
+  }
+  if (this.props.isRead) {
+    classString += ' message-read';
+  }
+  // 'message message-important message-read'
+  return <div className={classString}>Great, I'll be there.</div>
+}
+```
+
+This can quickly get tedious, as assigning class name strings can be hard to read and error-prone. `classSet()` solves this problem:
+
+```javascript
+render: function() {
+  var classSet = React.addons.classSet;
+  var classes = {
+    'message': true,
+    'message-important': this.props.isImportant,
+    'message-read': this.props.isRead
+  }
+  // same final string, but much cleaner
+  return <div className={classSet(classes)}>Great, I'll be there.</div>
+}
+```
+
+Pass to `classSet()` an object, whose key is the CSS class name you might or might not need, and whose value is a boolean (or anything that converts to it) that indicates whether the key should be present in the final class string.
+
+No more hacky string concatenations!