Add info about structs in the encode docs

[williamthome]

The encode documentation misses that structs are handled by the encode_map function as:

  defp encode_map(struct, state) when is_map_key(struct, :__struct__) do
    :euneus_encoder.encode_map(Map.from_struct(struct), state)
  end

  defp encode_map(map, state) do
    :euneus_encoder.encode_map(map, state)
  end

i.e., structs are encoded as Map.from_struct(struct).