title: ImproperlyConfiguredError about app_name when using namespace in include()
body: <p>I am currently trying out Django. I use the <code>namespace</code> argument in one of my <code>include()</code>s in urls.py. When I run the server and try to browse,
  I get this error.</p>
  <pre><code>File &quot;C:\Users\User\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\urls\conf.py&quot;, line 39, in include
      'Specifying a namespace in include() without providing an app_name '
  django.core.exceptions.ImproperlyConfigured: Specifying a namespace in include() without providing an app_name is not supported. Set the app_name attribute in the included module, or pass a 2-tuple containing the list of patterns and app_name instead.
  </code></pre>
  <p>These are my urls.py files:</p>
  <pre><code>#project/urls.py
  
  from django.conf.urls import include, url
  from django.contrib import admin
  
  urlpatterns = [
      url(r'^reviews/', include('reviews.urls', namespace='reviews')),
      url(r'^admin/', include(admin.site.urls)),
  ]
  </code></pre>
  <p>and</p>
  <pre><code>#app/urls.py
  
  from django.conf.urls import url
  
  from . import views
  
  urlpatterns = [
      # ex: /
      url(r'^$', views.review_list, name='review_list'),
      # ex: /review/5/
      url(r'^review/(?P&lt;review_id&gt;[0-9]+)/$', views.review_detail, name='review_detail'),
      # ex: /wine/
      url(r'^wine$', views.wine_list, name='wine_list'),
      # ex: /wine/5/
      url(r'^wine/(?P&lt;wine_id&gt;[0-9]+)/$', views.wine_detail, name='wine_detail'),
  ]
  </code></pre>
  <p>What do I pass the <code>app_name</code> as stated in the error message?</p>