Documentation: add benchmark to performance.md
Documentation: add benchmark to performance.md

Fix https://github.com/coreos/etcd/issues/5433.

mqliang/etcd,ajityagaty/etcd,luxas/etcd,whuwxl/etcd,sebrandon1/etcd,nekto0n/etcd,chipironcin/etcd,gyuho/etcd,rchicoli/etcd,sofuture/etcd,OyTao/etcd_learning,fanminshi/etcd,lumjjb/etcd,linuxcer/etcd,mqliang/etcd,endocode/etcd,jonboulle/etcd,mkumatag/etcd,xiang90/etcd,lorneli/etcd,purpleidea/etcd,chipironcin/etcd,nekto0n/etcd,qqsun8819/etcd,YuleiXiao/etcd,dannysauer/etcd,visheshnp/etcd,westhood/etcd,mkumatag/etcd,bluebreezecf/etcd,glycerine/etcd,linuxcer/etcd,CrazyJvm/etcd,rnaveiras/etcd,swingbach/etcd,rnaveiras/etcd,mickep76/etcd,coreos/etcd,tguangch/etcd,swingbach/etcd,armstrongli/etcd,jlamillan/etcd,akauppi/etcd,insequent/etcd,wantonsolutions/dinv-etcd,hongchaodeng/etcd,heyitsanthony/etcd,mqliang/etcd,a-robinson/etcd,hongchaodeng/etcd,zaphod-concur/etcd,ravigadde/etcd,a-robinson/etcd,mkumatag/etcd,purpleidea/etcd,gyuho/etcd,sinsharat/etcd,vimalk78/etcd,plasticbox/etcd,armstrongli/etcd,luxas/etcd,whuwxl/etcd,xiang90/etcd,ajityagaty/etcd,joshix/etcd,lumjjb/etcd,etcd-io/etcd,yudai/etcd,bdarnell/etcd,zaphod-concur/etcd,ktateish/etcd,cockroachdb/etcd,YuleiXiao/etcd,insequent/etcd,etcd-io/etcd,handsomesun/etcd,handsomesun/etcd,dannysauer/etcd,rchicoli/etcd,lorneli/etcd,wantonsolutions/dinv-etcd,glevand/coreos--etcd,jlamillan/etcd,sinsharat/etcd,StorReduce/etcd,glycerine/etcd,ajityagaty/etcd,etcd-io/etcd,cockroachdb/etcd,glevand/coreos--etcd,jonboulle/etcd,tguangch/etcd,ktateish/etcd,qqsun8819/etcd,OyTao/etcd_learning,vimalk78/etcd,sofuture/etcd,bdarnell/etcd,joshix/etcd,ravigadde/etcd,yudai/etcd,wantonsolutions/dinv-etcd,coreos/etcd,li-ang/etcd,endocode/etcd,fanminshi/etcd,akauppi/etcd,rnaveiras/etcd,westhood/etcd,li-ang/etcd,visheshnp/etcd,CrazyJvm/etcd,mickep76/etcd,sebrandon1/etcd,plasticbox/etcd,StorReduce/etcd,heyitsanthony/etcd,bluebreezecf/etcd