Add docs on request.accepted_renderer and .accepted_media_type

docs/api-guide/requests.md

@@ -49,6 +49,20 @@ If a client sends a request with a content-type that cannot be parsed then a `Un
 
 ---
 
+# Content negotiation
+
+The request exposes some properties that allow you to determine the result of the content negotiation stage. This allows you to implement behaviour such as selecting a different serialisation schemes for different media types.
+
+## .accepted_renderer
+
+The renderer instance what was selected by the content negotiation stage.
+
+## .accepted_media_type
+
+A string representing the media type that was accepted by the content negotiation stage.
+
+---
+
 # Authentication
 
 REST framework provides flexible, per-request authentication, that gives you the ability to: