Document cron job file naming restrictions

Cron job file names need to avoid . and + and should use _ for those
characters instead.  Also document a convention for packages that need
to include multiple cron job files.

Wording: Karl E. Jorgensen <karl@jorgensen.org.uk>
Wording: Russ Allbery <rra@debian.org>
Seconded: Russ Allbery <rra@debian.org>
Seconded: Dominic Hargreaves <dom@earth.li>
Seconded: Javier Fernández-Sanguino Peña <jfs@computer.org>
Closes: #609162