Helps avoid a common Issue when following tutorial

reactjs/react-tutorial#87

docs/docs/tutorial.md

@@ -130,6 +130,8 @@ You do not have to return basic HTML. You can return a tree of components that y
 
 The `ReactDOM` module exposes DOM-specific methods, while `React` has the core tools shared by React on different platforms (e.g., [React Native](http://facebook.github.io/react-native/)).
 
+It is important that `ReactDOM.render` remain at the bottom of the script for this tutorial. `ReactDOM.render` should only be called after the composite components have been defined.
+
 ## Composing components
 
 Let's build skeletons for `CommentList` and `CommentForm` which will, again, be simple `<div>`s. Add these two components to your file, keeping the existing `CommentBox` declaration and `ReactDOM.render` call: