evolve() and attr.ib(init=False, default="foo") #251

As pointed out in ericvsmith/dataclasses#42 (comment) attr.evolve() behaves rather weird if an attribute has

1. init=False,
2. a default/__attrs_post_init__, and
3. attribute(s) are modified on the instance.

The new resulting object will always have the default value.

I think I can live with that limitation if we document it and it's probably a good reason to resolve #207 with sticking assoc around with a better name?

Giving it further thought, the raison d'être for evolve are immutable objects…so I think it's fine to just document this edge case and leave it be?