Feature request - put in an example of how to use Tuple in docs

[codekiln]

How should we use typing.Tuple? Here's an ipython session illustrating the problem:

In [1]: from dataclasses_json import DataClassJsonMixin
In [2]: from dataclasses import dataclass

In [3]: @dataclass
    ...: class Foo(DataClassJsonMixin):
    ...:     bar: Tuple[Optional[float]]
    ...:

In [4]: Foo.schema()
ValueError: tuple_fields must be an iterable of Field classes or instances.

In [5]: @dataclass
    ...: class Foo(DataClassJsonMixin):
    ...:     bar: Tuple[Optional[float], Optional[float]]
    ...:

In [6]: Foo.schema()
TypeError: __init__() takes 1 positional argument but 2 were given