[1.8.x] Added each_context() and a template tip to custom admin view docs.

Backport of 648e516b8d from master

docs/ref/contrib/admin/index.txt

@@ -1440,8 +1440,23 @@ templates used by the :class:`ModelAdmin` views:
                 return my_urls + urls
 
             def my_view(self, request):
-                # custom view which should return an HttpResponse
-                pass
+                # ...
+                context = dict(
+                   # Include common variables for rendering the admin template.
+                   self.each_context(request),
+                   # Anything else you want in the context...
+                   key=value,
+                )
+                return TemplateResponse(request, "sometemplate.html", context)
+
+    If you want to use the admin layout, extend from ``admin/base_site.html``:
+
+    .. code-block:: html+django
+
+       {% extends "admin/base_site.html" %}
+       {% block content %}
+       ...
+       {% endblock %}
 
     .. note::