Stringify argument not documented.

[abberadhi]

I encountered an issue that left me scratching my head for a good while... When stringifying a cron expression that contains 6 fields accoring to the readme, the result is 5 fields (missing seconds).

It turns out .stringify() takes a boolean argument that controls whether to include the second field.

It would be good to have this information included in the readme.

Thanks,
A.