Need documentation for supported cron expressions

[mTsBucy1]

I was trying to use this crate to parse crontab expression as generated by https://crontab.guru/ or as documented by wikipedia article https://en.wikipedia.org/wiki/Cron#Cron_expression. In particular do neither of them include the second field. However, this crate accepts only expressions with 6 or 7 field (following the Oracle conventions https://docs.oracle.com/cd/E12058_01/doc/doc.1014/e12030/cron_expressions.htm). This discrepancy should at least be documented. Other non-standard extensions, such as the optional year and the @ shorthands should also be mentioned.

Addtionally, I would still like to use the second-less expression and I've noticed there was once an issue #5 about the optional seconds field, I guess it was dropped in favour of the optional year field. Is there any possibility of support for this adding this?

Thanks for your work!

[mTsBucy1]

I've noticed this is essentially a duplicate of #37 (comment), sorry for this, but I would still appreciate a quick README / Docs.rs comment.