What's the task? Please describe

https://github.com/longhorn/longhorn-spdk-engine has an outdated tutorial to guide people on how to prepare and set up the environment for using SPDK, run new Longhorn SPDK bdev, etc.

Need to update the tutorial because right now we already decide to use RAID1 bdev to achieve our new Longhorn SPDK volume.

Describe the items of the task (DoD, definition of done) you'd like

• Update readme to guide users on how to setup the environment for running RAID1

Additional context

cc @longhorn/dev-data-plane