add tip for the important jsx convention

I missed the convention and wast hours to figure out what's wrong. 
Add this tip in the document for who would miss the importance comment.

docs/getting-started.md

@@ -57,6 +57,8 @@ React.renderComponent(
   document.getElementById('example')
 );
 ```
+> ```/** @jsx React.DOM */``` is must, or jsx would not convert.
+
 Then reference it from `helloworld.html`:
 
 ```html{10}