flesh up the example

t/example.t

@@ -66,10 +66,15 @@ $registry->add_type(
             self => sub($data,@) {
                 return "/articles/" . $data->{id};
             },
+        },
+        relationships => {
+            'tags' => { type => 'tag' },
         }
     }
 );
 
+$registry->add_type('tag');
+
 my $output = $registry->serialize('article', $data, { count => 2 } );
 
 like $output => {
@@ -83,6 +88,26 @@ like $output => {
   "links"=> {
     "self"=> "/articles"
   },
+  "data"=> [{
+    "type"=> "article",
+    "id"=> "1",
+    "attributes"=> {
+      "title"=> "JSON API paints my bikeshed!",
+      "body"=> "The shortest article. Ever.",
+      "created"=> "2015-05-22T14:56:29.000Z"
+    },
+    "relationships"=> {
+      "tags"=> {
+        "data"=> [{
+          "type"=> "tag",
+          "id"=> "1"
+        }, {
+          "type"=> "tag",
+          "id"=> "2"
+        }]
+      },
+    }
+   }],
 };
 
 done_testing();