Save copies of add request messages

To allow debugging why errors were found in seemingly good entries that
were sent, save a copy of all messages that request adding an entry.
Previously no records were kept of files which were found to contain
errors.
Ignore some new files
Fix description of date format

State that full four-digit year should be used, rather than the
two-digits that were used in the LSM3 format which was replaced in 1999.

While modifying that, also insert a couple of "the"s to resolve some
grammar issues.

Reported-by: Gilles LAMIRAL <gilles.lamiral@laposte.net>
Ignore add-requests directory

Currently storing incoming add request messages in add-requests maildir,
have that ignored by git.