Tell user to keep the @jsx declaration at the top of the script

Working through the tutorial, I added some code above the

```
      /**
       * @jsx React.DOM
       */
```

declaration, and this resulted in a very unhelpful error message from JSXTransformer:

```
Uncaught SyntaxError: Unexpected token <
851547_221914424655081_22271_n.js:10217
```

Hopefully the comment will convince a few people to keep the declaration at the very beginning.