doc: README.iscsi: Open-iSCSI configuration

Provide settings for Open-iSCSI

Reformat headers. h3-headers marked with ^^^ are not recognized in some
markup editors. Use the ### notation instead.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>