Add warning about module.exports use (#505)

* add warning about module.exports to readme

* rm unnecessary react imports

README.md

@@ -477,6 +477,29 @@ module.exports = {
 }
 ```
 
+#### Default Babel configuration
+Next.js uses Babel6 out of the box, which doesn't support default `module.exports`.
+##### Invalid:
+```js
+//  bad import
+const Link = require('next/link').default;
+
+//  bad export
+module.exports = () => (
+  <Link href='/'>Invalid</Link>
+)
+```
+##### Valid:
+```js
+//  good import
+import Link from 'next/link';
+
+//  good export
+export default () => (
+  <Link href='/'>Valid</Link>
+)
+```
+
 ## Production deployment
 
 To deploy, instead of running `next`, you probably want to build ahead of time. Therefore, building and starting are separate commands: