Update README for proper eager loading

From my experience with your package this is the only proper way to eager load. Otherwise the script would query the DB for every TAG I have. Test it yourself and let me know.

README.md

@@ -45,7 +45,10 @@ class Article extends \Illuminate\Database\Eloquent\Model {
 #### Quick Sample Usage
 
 ```php
-$article = Article::with('tagged')->first(); // eager load
+$article = Article::with(['tagged' => function($query) {
+                    $query->with('tag');
+                }])
+                ->first(); // eager load
 
 foreach($article->tagged as $tagged) {
 	echo $tagged->tag_name . ' with url slug of ' . $tagged->tag_slug;