SerializerMethodField.read_only is True by default

[djshen-ponddy]

https://github.com/tomchristie/django-rest-framework/blob/300e46a9e5798ca56f56ba231de3d3fded860551/rest_framework/fields.py#L1722

We don't need to declare the field names of SerializerMethodField instances in read_only_fields