op-guide: add notes on mounting certs directory
op-guide: add notes on mounting certs directory

Signed-off-by: Gyu-Ho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

whuwxl/etcd,bdarnell/etcd,heyitsanthony/etcd,gyuho/etcd,jlamillan/etcd,heyitsanthony/etcd,lorneli/etcd,gyuho/etcd,etcd-io/etcd,li-ang/etcd,a-robinson/etcd,rnaveiras/etcd,jlamillan/etcd,fanminshi/etcd,lorneli/etcd,visheshnp/etcd,CrazyJvm/etcd,etcd-io/etcd,lumjjb/etcd,CrazyJvm/etcd,tguangch/etcd,glevand/coreos--etcd,bluebreezecf/etcd,joshix/etcd,yudai/etcd,sinsharat/etcd,rnaveiras/etcd,handsomesun/etcd,mickep76/etcd,cockroachdb/etcd,bdarnell/etcd,lumjjb/etcd,li-ang/etcd,mqliang/etcd,armstrongli/etcd,mqliang/etcd,bluebreezecf/etcd,YuleiXiao/etcd,mkumatag/etcd,armstrongli/etcd,mqliang/etcd,tguangch/etcd,sinsharat/etcd,rnaveiras/etcd,vimalk78/etcd,a-robinson/etcd,visheshnp/etcd,YuleiXiao/etcd,dannysauer/etcd,mkumatag/etcd,xiang90/etcd,plasticbox/etcd,akauppi/etcd,cockroachdb/etcd,ajityagaty/etcd,coreos/etcd,xiang90/etcd,coreos/etcd,ajityagaty/etcd,sebrandon1/etcd,dannysauer/etcd,plasticbox/etcd,mickep76/etcd,vimalk78/etcd,sebrandon1/etcd,handsomesun/etcd,etcd-io/etcd,fanminshi/etcd,ajityagaty/etcd,whuwxl/etcd,mkumatag/etcd,yudai/etcd,akauppi/etcd,joshix/etcd,glevand/coreos--etcd