docs(tutorial): added a link to Wiki page on DI in Step 7

This article is fantastic and really helped on understanding how DI works on Angular. It may be
useful to other beginners -- because, at first glance, this topic (DI on Angular) ended a little bit
hazy for me.

Closes angular#7010

docs/content/tutorial/step_07.ngdoc

@@ -98,6 +98,9 @@ of configuring the injector. As opposed to AMD or require.js modules, Angular mo
 solve the problem of script load ordering or lazy script fetching. These goals are totally independent and
 both module systems can live side by side and fulfil their goals.
 
+For deepen your understanding of DI on Angular, see
+[Understanding Dependency Injection](https://github.com/angular/angular.js/wiki/Understanding-Dependency-Injection).
+
 ## Template
 
 The `$route` service is usually used in conjunction with the {@link ngRoute.directive:ngView