document how to convert any item to a dict

[kmike]

I think that'd be helpful to explain that dict(ItemAdapter(item)) gives you a plain version of an item; it could also make sense to add a explicit test for it.

[elacuesta]

Docs added in d018404, tests available at https://github.com/scrapy/itemadapter/blob/v0.0.6/tests/test_adapter.py#L149-L153