Update 09.1-animation.md

The reason I chose to submit this is because it was initially unclear from the docs that the transition group needed to already be mounted in order for it to work properly.

docs/09.1-animation.md

@@ -83,6 +83,30 @@ You'll notice that when you try to remove an item `ReactCSSTransitionGroup` keep
 }
 ```
 
+### Animation Group Must Be Mounted To Work
+
+In order for it to apply transitions to its children, the `ReactCSSTransitionGroup` must already be mounted in the DOM. The example below would not work, because the `ReactCSSTransitionGroup` is being mounted along with the new item, instead of the new item being mounted within it. Compare this to the [Getting Started](/docs/docs/09.1-animation.md#getting-started) section above to see the difference.
+
+```javascript{12-14}
+render: function() {
+    var items = this.state.items.map(function(item, i) {
+      return (
+        <div key={item} onClick={this.handleRemove.bind(this, i)}>
+          <ReactCSSTransitionGroup transitionName="example">
+            {item}
+          </ReactCSSTransitionGroup>
+        </div>
+      );
+    }.bind(this));
+    return (
+      <div>
+        <button onClick={this.handleAdd}>Add Item</button>
+        {items}
+      </div>
+    );
+  }
+```
+
 ### Animating One or Zero Items
 
 Although in the example above we rendered a list of items into `ReactCSSTransitionGroup`, the children of `ReactCSSTransitionGroup` can be one or zero items. This makes it possible to animate a single element entering or leaving. Similarly, you can animate a new element replacing the current element, with the new element animating in while the current animates out. For example, we can implement a simple image carousel like this: