isPreview defaults to true

[jessecascio]

Theres logic in there that overrides the default Apple News API behaviour with no documentation that this happens. As outlined in the Apple docs on isPreview:

Default value: True if your channel has not yet been approved to publish articles in Apple News Format; False if your channel has been approved.

But this module explicitly sets it to true if not defined.

[iefserge]

@jessecascio Yep, I think this way it's more predictable and requires explicit permission to publish public articles.

[ryanscottaudio]

Added isPreview = true to readme.