Add note to Date and Time custom types indicating acceptable values

[paradox460]

Add a note to the Date and Time custom types indicating that they do
not accept Time and date values respectively.

Date.from_iso8601("2019-04-15T11:55:15")
# => {:error, :invalid_format}

Date.from_iso8601("2019-04-15")
# => {:ok, ~D[2019-04-15]}

Time.from_iso8601("2019-04-15T11:55:15")
# => {:error, :invalid_format}

Time.from_iso8601("11:55:15")
# => {:ok, ~T[11:55:15]}

Some platforms, such as iOS Apollo, cast Date/Time to types native to that platform. Swift Date accepts full ISO8601:2019 datetime stamps.

[bruce]

Good and helpful distinctions, thank you Jeff!

[paradox460]

Happy to help!