Document include options for Compound Documents

With #250 pulled in we now have options to include relations to form compound documents:

1. Include resources related to the primary data by default.
2. Include resources related to the primary data through use of a request parameter which customizes which related resources should be returned

Example Current Implementation

1. default include posts

class UserSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=200)
    # PostSerializer included directly on the serializer
    posts = PostSerializer(many=True, read_only=True)

    class JSONAPIMeta:
        # included_resources specifys resources to include by default
        included_resources = ['posts']

1. param include posts

class UserSerializer(serializers.Serializer):
    name = serializers.CharField(max_length=200)
    # PostSerializer NOT included directly on the serializer, ResourceRelatedField is 
    # specified as default
    posts = relations.ResourceRelatedField(
            source='post_set', many=True, read_only=True) 

    # specifies serializer to use when passing in the `include` param
    included_serializers = {
        'posts': PostSerializer,
    }

We should probably document this. We can even copy the examples above with a bit of explanation.

[mblayman]

Since this was still open, I took a crack at documenting it in PR #308.

[timmyomahony]

Thanks for this. I had to find this post to make sense of included records. It would be great to get some official documentation for this. It’s a common pattern for sideloading data in Ember for example