Update Readme with ixsystems_portal_id and ixsystems_initiator_id

[randifilan]

When you not use ixsystems_portal_id and ixsystems_initiator_id, the default value for the first create is 1.
When your ixsystems_portal_id and ixsystems_initiator_id is not 1, you must change this value that present ID in your truenas, otherwise it will be error.

[ixsystems-iscsi]
ixsystems_portal_id = 2
ixsystems_initiator_id = 2