Added docs about the ref-as-callback.

docs/docs/08.1-more-about-refs.md

@@ -65,7 +65,7 @@ In this counterexample, the `<input />` is merely a *description* of an `<input
 
 So how do we talk to the *real* backing instance of the input?
 
-## The ref Attribute
+## The ref String Attribute
 
 React supports a very special property that you can attach to any component that is output from `render()`. This special property allows you to refer to the corresponding **backing instance** of anything returned from `render()`. It is always guaranteed to be the proper instance, at any point in time.
 
@@ -85,6 +85,18 @@ It's as simple as:
 
   You can access the component's DOM node directly by calling `React.findDOMNode(this.refs.myInput)`.
 
+
+## The ref Callback Attribute
+
+The `ref` attribute can be a callback function instead of a name.  This callback will be executed immediately after the component is mounted.  The referenced component will be passed in as a parameter, and the callback function may use the component immediately, or save the reference for future use (or both).
+
+It's as simple as assigning a `ref` attribute to anything returned from `render` such as:
+
+  ```html
+  <input ref={ function(component){ React.findDOMNode(component).focus();} } />
+  ```
+
+
 ## Completing the Example
 
 ```javascript