Adding examples and documentation for caching

The default caching system of Django works very well with DRF. However, the usage is not very easy to understand for class based views as the documentation is spread across pages. This pull-request provides example code on how to use caching with apiviews and viewsets.

docs/api-guide/caching.md

@@ -0,0 +1,54 @@
+# Caching
+
+> A certain woman had a very sharp conciousness but almost no 
+> memory ... She remembered enough to work, and she worked hard.
+> - Lydia Davis
+
+Caching in REST Framework works well with the cache utilities
+provided in Django.
+
+---
+
+## Using cache with apiview and viewsets
+
+Django provides a [`method_decorator`][decorator] to use
+decorators with class based views. This can be used with
+with other cache decorators such as [`cache_page`][page] and
+[`vary_on_cookie`][cookie].
+
+```python
+from rest_framework.response import Response
+from rest_framework.views import APIView
+from rest_framework import viewsets
+
+class UserViewSet(viewsets.Viewset):
+    
+    # Cache requested url for each user for 2 hours
+    @method_decorator(cache_page(60*60*2))  
+    @method_decorator(vary_on_cookie)
+    def list(self, request, format=None):
+        content = {
+            'user_feed': request.user.get_user_feed()
+        }
+        return Response(content)
+
+class PostView(APIView):
+
+    # Cache page for the requested url
+    @method_decorator(cache_page(60*60*2))  
+    def get(self, request, format=None):
+        content = {
+            'title': 'Post title',
+            'body': 'Post content'
+        }
+        return Response(content)
+```
+
+**NOTE:** The [`cache_page`][page] decorator only caches the
+`GET` and `HEAD` responses with status 200.
+
+
+[django]: https://docs.djangoproject.com/en/dev/topics/cache/
+[page]: https://docs.djangoproject.com/en/dev/topics/cache/#the-per-view-cache
+[cookie]: https://docs.djangoproject.com/en/dev/topics/http/decorators/#django.views.decorators.vary.vary_on_cookie
+[decorator]: https://docs.djangoproject.com/en/dev/topics/class-based-views/intro/#decorating-the-class