adding description of zfs setup done in a database machine

c3sldb1_zfs_setup.md

+# ZFS config c3sldb1
+## Storage pool
+
+Configured 6 disks of 6Tb each on a RAID6 setup (raidz2 as called by ZFS)
+ 
+`zpool create tank raidz2 -f <6TB_DISKS_IDS>`
+
+## Caches
+### Read cache devices
+Two 300Gb SSDs with striped data
+`zpool add tank cache <300GB_SSDS_IDS>`
+
+### Write cache devices
+Two 240Gb SSDs mirroed in a l2arc setup. If one fails, there is another to cover the data writes.
+
+`zpool add tank log mirror <240GB_SSDS_IDS>`