Documenting parsing metadata in request body

Author: Hamed Ahmadi

docs/api.md

@@ -47,3 +47,14 @@ Calls a `get_root_meta` function on a serializer, if it exists.
 `build_json_resource_obj(fields, resource, resource_instance, resource_name)`
 
 Builds the resource object (type, id, attributes) and extracts relationships.
+
+## rest_framework_json_api.parsers.JSONParser
+
+Similar to `JSONRenderer`, the `JSONParser` you may override the following methods if you need
+highly custom parsing control.
+
+#### parse_metadata
+
+`parse_metadata(result)`
+
+Returns a dictionary which will be merged into parsed data of the request. By default, it reads the `meta` content in the request body and returns it in a dictionary with a `_meta` top level key.

docs/usage.md

@@ -375,7 +375,7 @@ class LineItemViewSet(viewsets.ModelViewSet):
 
 ### RelationshipView
 `rest_framework_json_api.views.RelationshipView` is used to build
-relationship views (see the 
+relationship views (see the
 [JSON API spec](http://jsonapi.org/format/#fetching-relationships)).
 The `self` link on a relationship object should point to the corresponding
 relationship view.
@@ -449,6 +449,8 @@ def get_root_meta(self, resource, many):
 ```
 to the serializer. It must return a dict and will be merged with the existing top level `meta`.
 
+To access metadata in incoming requests, the `JSONParser` will add the metadata under a top level `_meta` key in the parsed data dictionary. For instance, to access meta data from a `serilizer` object, you may use `serializer.initial_data.get("_meta")`. To customize the `_meta` key, see [here](api.md).
+
 ### Links
 
 Adding `url` to `fields` on a serializer will add a `self` link to the `links` key.