Document how to set home directory

The admin check is already there, so adding documentation how to change
the setting should be enough. We don't default to using another
directory as this would be migration nightmare for existing setups.

Closes #493, #596

Signed-off-by: Michal Čihař <michal@cihar.com>
Warn user if home is not writable (issue #493)

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