zpool: added cache device example (#2715)

pages/common/zpool.md

@@ -29,3 +29,7 @@
 - Create a mirrored pool:
 
 `zpool create {{pool_name}} mirror {{disk1}} {{disk2}} mirror {{disk3}} {{disk4}}`
+
+- Add a cache (L2ARC) device to a zpool:
+
+`zpool add {{pool_name}} cache {{cache_disk}}`