doc: add upgrade to 2.2 doc
doc: add upgrade to 2.2 doc

fasaxc/etcd,onlyjob/etcd,tguangch/etcd,dannysauer/etcd,hongchaodeng/etcd,arthurcolle/etcd,StorReduce/etcd,beni55/etcd,mkumatag/etcd,mkumatag/etcd,fgbreel/etcd,mqliang/etcd,sebrandon1/etcd,jlamillan/etcd,iamqizhao/etcd,arthurcolle/etcd,improbable-io/etcd,HideoYamauchi/etcd,wowsoso/etcd,akauppi/etcd,xdlinear/etcd,jeremyeder/etcd,plasticbox/etcd,oscarrr110/etcd,tguangch/etcd,pauldallen/etcd,mickep76/etcd,a-robinson/etcd,rnaveiras/etcd,StorReduce/etcd,zaphod-concur/etcd,gyuho/etcd,fanminshi/etcd,akauppi/etcd,combk8s/etcd,gityf/etcd,ktateish/etcd,lorneli/etcd,glevand/coreos--etcd,lumjjb/etcd,asim/etcd,raoofm/etcd,Yhgenomics/etcd,cchamplin/etcd,nekto0n/etcd,xychu/etcd,tamird/etcd,mike-lerch/etcd,OyTao/etcd_learning,hroyrh/etcd,HideoYamauchi/etcd,swingbach/etcd,zaphod-concur/etcd,chenbaihu/etcd,yudai/etcd,insequent/etcd,armstrongli/etcd,ktateish/etcd,xdlinear/etcd,bluebreezecf/etcd,msanterre/etcd,sofuture/etcd,bdarnell/etcd,siddontang/etcd,mike-lerch/etcd,beni55/etcd,YuleiXiao/etcd,sinsharat/etcd,joshix/etcd,dnaeon/etcd,siddontang/etcd,TheHippo/etcd,jchauncey/etcd,fasaxc/etcd,rchicoli/etcd,westhood/etcd,handsomesun/etcd,purpleidea/etcd,rnaveiras/etcd,kkaneda/etcd,achanda/etcd,cockroachdb/etcd,etcd-io/etcd,chipironcin/etcd,CrazyJvm/etcd,yudai/etcd,sofuture/etcd,Winslett/etcd,bdarnell/etcd,hongchaodeng/etcd,chipironcin/etcd,jonboulle/etcd,Eagle-X/etcd,mkumatag/etcd,cockroachdb/etcd,insequent/etcd,mqliang/etcd,visheshnp/etcd,gouyang/etcd,glycerine/etcd,ajityagaty/etcd,sinsharat/etcd,jlamillan/etcd,Timer/etcd,gyuho/etcd,improbable-io/etcd,rnaveiras/etcd,sayden/etcd,dannysauer/etcd,wantonsolutions/dinv-etcd,YuleiXiao/etcd,sayden/etcd,gityf/etcd,cchamplin/etcd,fgbreel/etcd,giantswarm/etcd,delkyd/etcd,glycerine/etcd,glevand/coreos--etcd,wantonsolutions/dinv-etcd,pauldallen/etcd,heyitsanthony/etcd,xiang90/etcd,gae123/etcd,endocode/etcd,whuwxl/etcd,iamqizhao/etcd,ravigadde/etcd,shawnps/etcd,Timer/etcd,hroyrh/etcd,etcd-io/etcd,jeremyeder/etcd,mqliang/etcd,giantswarm/etcd,svdata/etcd,kkaneda/etcd,mickep76/etcd,handsomesun/etcd,jchauncey/etcd,yichengq/etcd,jonboulle/etcd,jefby/etcd,coreos/etcd,endocode/etcd,a-robinson/etcd,armstrongli/etcd,lumjjb/etcd,westhood/etcd,fasaxc/etcd,linuxcer/etcd,purpleidea/etcd,svdata/etcd,xychu/etcd,dnaeon/etcd,oscarrr110/etcd,heyitsanthony/etcd,qqsun8819/etcd,Yhgenomics/etcd,msanterre/etcd,whuwxl/etcd,rchicoli/etcd,bluebreezecf/etcd,gae123/etcd,tamird/etcd,delkyd/etcd,etcd-io/etcd,betawaffle/etcd,wantonsolutions/dinv-etcd,ajityagaty/etcd,combk8s/etcd,plasticbox/etcd,luxas/etcd,yichengq/etcd,vimalk78/etcd,asim/etcd,fanminshi/etcd,swingbach/etcd,visheshnp/etcd,hongbinz/etcd,sebrandon1/etcd,onlyjob/etcd,raoofm/etcd,qqsun8819/etcd,nekto0n/etcd,wowsoso/etcd,buaazp/etcd,ajityagaty/etcd,betawaffle/etcd,Eagle-X/etcd,achanda/etcd,OyTao/etcd_learning,jefby/etcd,lorneli/etcd,chenbaihu/etcd,vimalk78/etcd,shawnps/etcd,xiang90/etcd,gouyang/etcd,CrazyJvm/etcd,luxas/etcd,buaazp/etcd,hongbinz/etcd,li-ang/etcd,linuxcer/etcd,Winslett/etcd,joshix/etcd,ravigadde/etcd,li-ang/etcd,TheHippo/etcd,coreos/etcd