Add strftime formatting string to CDXJ index timestamp

[extua]

I found the strftime format for 14-digit CDXJ timestamps in the timeutils module of warcio.

%Y%m%d%H%M%S

It would be useful to mention this in the spec itself (or in an annotation).

[extua]

Looking up the strftime string is not difficult, but would make peoples lives easier to include it directly.