add documentation about cloning submodules

Author: songgao

dockerfile_tutorial/templates/dockerfile/level1.html

@@ -132,7 +132,7 @@ <h3>docker build</h3>
 </li>
 <li>
 	From GitHub. For instance: <code>docker build github.com/creack/docker-firefox</code><br />
-	In this last example, docker will clone the GitHub repository and use it as context. The Dockerfile at the root of the repository will be used to create the image.
+	In this last example, docker will clone the GitHub repository with its submodules (`git clone --recursive`) and use it as context. The Dockerfile at the root of the repository will be used to create the image.
 </li>
 </ol>
 <p>