README: a git-diff style example of how a group of serializers compares to serialization_spec

[jordaneb]

i reckon this would be really good for illustrating to people how to get their heads around how it works in comparison

e.g.

- class MySerializer(serializers.Serializer):
-     field = CharField()
-     other = SerializerMethodField()     
-
-     def get_other(self):
-         # something that can be replaced with a plugin
+ serialization_spec = [
+     'field',
+     {'other': OtherPlugin},
+ ]

[pmg103]

I see what you mean but SSM is so much simpler than hand-rolled serializer/view I feel like its sad to have to make reference to that more complicated thing to explain a simpler thing.

Maybe a supplementary migration guide would be useful though