Add a prerequisite section for disks used by cStor

[kmova]

Cover the prerequisites for disks like:

• Should they be unmounted or cleared of any older FS?
• Should they be partitioned or not?
• Can LVM disks be used?
• Can external SAN mounted disks be used?

[akhilerm]

• (Q) Should they be unmounted or cleared of any older FS? Yes, the disks should be unmounted and better it should be cleared of any file system, though there is an option -f that can be given which wipe out the file system on the disk. We have to check if control plane is using -f or not.
• (Q) Should they be partitioned or not? No. Partition is not supported yet.
• (Q) Can LVM disks be used? No. Currently we are not supporting LVMs. LVMs will be excluded by path filter.
• (Q) Can external SAN mounted disks be used? Yes. External SAN disks will be detected by NDM and can be used by cStor.