Allow to subclass a Field and make it read-only

Rather than subclass a `Field` and override `__init__` you may want a more data-driven approach where the defaults for `read_only`, `write_only` and `allow_null` are specified on the class.

e.g.
```python
class ReadOnlyLink(serializers.HyperlinkedRelatedField)
    read_only = True
```