Document that explicitly declared fields will not have their attributes overridden by `read_only_fields` shortcut.

``` python
class PersonSerializer(serializers.ModelSerializer):
    info = serializers.Field(source='info')

    class Meta:
        model = Person
        read_only_fields = ('info',)
```

In this case _read_only_fields_ is not working (info field is not read only) and we have to add _read_only=True_ to the field. It's very confusing.
