Document how to use nested JSON decoding to structs in more detail

[DaniruKun]

At the moment, there is practically no documentation on advanced usage of Poison.decode with the as option for decoding JSON to deeply nested structures with arrays of objects.

This needs at least one advanced example (either in the README or API docs).

[ivan-srsen]

You need to define model structure, like this:

def from_json(json) when is_binary(json),
do: json |> Poison.decode!(%{as: model_structure(), keys: :atoms})

defp model_structure do
%Model1{
model2: %Model2{
model3: %Model3{}
}
}
end```

It would be cool if it could decode a model based on a typespec.