Add item to "What exactly does edtf-validate do?" section of the README

[mjordan]

This library performs (at least in some cases) validation of dates against the Gregorian calendar. For example, 1930-02-34 fails validation as does 2023-02-29 (non leap year), even though these dates comply with the EDTF specification.

If this inference is correct (I admit to not tracking down this validation in the code), it might be useful to add this to the README, especially since the specification is silent about validating against specific calendars.

[DonRichards]

While the current error message provides information about the location and nature of the error, it doesn't specify the reason the date is considered invalid. To make the error message more user-friendly and helpful, you could clarify that the date validation is against the Gregorian calendar if I'm understanding your comments completely.

Here's a suggested reworded error message:

Error: The value "1930-02-34" in field "field_date_published" (row with ID 1030) is not recognized as a valid date in the Gregorian calendar, even though it may comply with the EDTF specification. Please ensure dates match the Gregorian calendar format.

Or

Error: Value in field "field_date_published" in row with ID 1030 ("1930-02-34") is not a valid EDTF date/time. Please see http://docs... for more information on possible causes of a invalidation trigger.