Update 10.1-animation.md

Custom classes

docs/docs/10.1-animation.md

@@ -84,6 +84,32 @@ You'll notice that when you try to remove an item `ReactCSSTransitionGroup` keep
 }
 ```
 
+### Custom Classes ###
+It is also possible to use custom class names for each of the steps in your transitions. Instead of passing a string into transitionName you can pass an object containing either the `enter` and `leave` class names, or an object containing the `enter`, `enter-active`, `leave-active`, and `leave` class names. If only the enter and leave classes are provided, the enter-active and leave-active classes will be determined by appending '-active' to the end of the class name. Here are two examples using custom classes:
+
+```javascript
+  ...
+  <ReactCSSTransitionGroup
+    transitionName={
+      enter: 'enter',
+      enterActive: 'enterActive',
+      leave: 'leave',
+      leaveActive: 'leaveActive'
+    }>
+    {item}
+  </ReactCSSTransitionGroup>
+  
+  <ReactCSSTransitionGroup
+    transitionName={
+      enter: 'enter',
+      leave: 'leave'
+    }>
+    {item2}
+  </ReactCSSTransitionGroup>
+  ...
+    
+```
+
 ### 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](#getting-started) section above to see the difference.