Add crontab instructrions.

Add crontab instructrions.

Too many people have been confused about crontab dependent functions
"not working" automatically on their installations.  This is probably
because there was no explicit step telling them to schedule the jobs.
The maintainers of the other INSTALL.* docs would be well advised
to insert similar lines in their docs, after confirming proper operation,
since cron/crontab are very system dependent.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>