Update AMP docs (#11353)

* Add caveat about css support for amp

* Update docs/advanced-features/amp-support/introduction.md

* Update introduction.md

Co-authored-by: Shu Uesugi <shu@chibicode.com>
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>

docs/advanced-features/amp-support/introduction.md

@@ -19,6 +19,10 @@ You can read more about AMP in the official [amp.dev](https://amp.dev/) site.
 
 To enable AMP support for a page, and to learn more about the different AMP configs, read the [API documentation for `next/amp`](/docs/api-reference/next/amp.md).
 
+## Caveats
+
+- Only CSS-in-JS is supported. [CSS Modules](/docs/basic-features/built-in-css-support.md) aren't supported by AMP pages at the moment. You can [contribute CSS Modules support to Next.js](https://github.com/zeit/next.js/issues/10549).
+
 ## Related
 
 For more information on what to do next, we recommend the following sections: