Update dependencies installation instructions for Debian

Signed-off-by: Michal Čihař <michal@cihar.com>
Document database setup

Issue #602

Signed-off-by: Michal Čihař <michal@cihar.com>
Document first steps

Signed-off-by: Michal Čihař <michal@cihar.com>
Document secret key generation

Signed-off-by: Michal Čihař <michal@cihar.com>
Report that cron job is not running

Signed-off-by: Michal Čihař <michal@cihar.com>