Add explanation for arguments passing

[magicpieh28]

I added explanation about how to run the class or function that needs arguments when using py>.

Currently, there is no explanation in the digdag documents, but actually we can pass arguments using _export. So I thought the explanation needed.

[yoyama]

@magicpieh28 Thank you for your contribution.

Based on the example https://github.com/treasure-data/digdag/blob/master/examples/python_args.dig It describe another way to pass argument. So if we add the description about the way to pass argument in doc, it might be better to describe both way.

[yoyama]

@magicpieh28 Thank you for your update. I hope add description about the way to pass argument as task parameters.

[magicpieh28]

@yoyama Please review my modification. Thank you.

[yoyama]

@magicpieh28 Sorry for my late reply. I left a comment. Could you take a look it ?