bug#80974: Add Time conversion specifiers quick reference

P=C3=A1draig Brady wrote:
> I should have confirmed the latest format currently committed,
> which is as follows

Thanks. Also I like that you have put the rarely used directives
(day of year, weekday, etc.) in a separate section, after the other
=E2=80=94 more frequently needed =E2=80=94 directives.

Bruno