feat(README): add note about cluster size
feat(README): add note about cluster size

Cluster size keeps coming up on IRC and the mailing list. Add an FAQ
section.

whuwxl/etcd,qqsun8819/etcd,lumjjb/etcd,hpcloud/etcd,Timer/etcd,fgbreel/etcd,mkumatag/etcd,xiang90/etcd,westhood/etcd,mwitkow-io/etcd,justintung/etcd,visheshnp/etcd,gouyang/etcd,spacejam/etcd,StorReduce/etcd,youxidev/etcd,shafreeck/etcd,arthurcolle/etcd,glevand/coreos--etcd,hongbinz/etcd,tempbottle/etcd,yyljlyy/etcd,gosharplite/etcd,wfxiang08/etcd,Timer/etcd,xebitstudios/etcd,imzhulei/etcd,rchicoli/etcd,hongbinz/etcd,yifan-gu/etcd,betawaffle/etcd,offscale/etcd,jlamillan/etcd,mwitkow/etcd,glensc/etcd,coreos/etcd,sofuture/etcd,mitake/etcd,devfreesg/etcd,fasaxc/etcd,nekto0n/etcd,pauldallen/etcd,tamird/etcd,tangkun75/etcd,onlyjob/etcd,etcd-io/etcd,wantonsolutions/dinv-etcd,mike-lerch/etcd,wfxiang08/etcd,armstrongli/etcd,ajityagaty/etcd,siddontang/etcd,bspline/etcd,combk8s/etcd,yudai/etcd,endocode/etcd,mischief/etcd,akauppi/etcd,jlamillan/etcd,xdlinear/etcd,giantswarm/etcd,dterei/etcd,rnaveiras/etcd,rnaveiras/etcd,chipironcin/etcd,es-chow/etcd,buaazp/etcd,glycerine/etcd,insionng/etcd,gae123/etcd,SivagnanamCiena/etcd,cloudfoundry/etcd,csuideal/etcd,linuxcer/etcd,mqliang/etcd,pengzj/etcd,betawaffle/etcd,yichengq/etcd,ktateish/etcd,steveeJ/etcd,barakmich/etcd,devfreesg/etcd,wantonsolutions/dinv-etcd,asim/etcd,qqsun8819/etcd,princeqjzh/etcd,beni55/etcd,kkaneda/etcd,rnaveiras/etcd,siddontang/etcd,gtank/etcd,steveeJ/etcd,mwitkow-io/etcd,Ulexus/etcd,mickep76/etcd,glensc/etcd,offscale/etcd,cvik/etcd,justintung/etcd,etcd-io/etcd,endocode/etcd,nkhare/etcd,cloudfoundry/etcd,asim/etcd,robszumski/etcd,chenbaihu/etcd,sayden/etcd,mkumatag/etcd,msanterre/etcd,lorneli/etcd,pauldallen/etcd,YuleiXiao/etcd,tguangch/etcd,wantonsolutions/dinv-etcd,cockroachdb/etcd,iamqizhao/etcd,chai2010/etcd,chai2010-playground/etcd,shakefu/etcd,ajityagaty/etcd,csuideal/etcd,qiaochao911/etcd,a-robinson/etcd,olark/etcd,xueshanf/etcd,ylong/etcd,sebschrader/etcd,robszumski/etcd,chai2010/etcd,vimalk78/etcd,yudai/etcd,mkumatag/etcd,es-chow/etcd,crandles/etcd,chenbaihu/etcd,improbable-io/etcd,purpleidea/etcd,openedbox/etcd,sayden/etcd,hongchaodeng/etcd,dterei/etcd,jonboulle/etcd,barakmich/etcd,sebschrader/etcd,spotify/etcd,freeloop1114/etcd,princeqjzh/etcd,fanminshi/etcd,lorneli/etcd,shawnps/etcd,cloudfoundry/etcd,shafreeck/etcd,nkhare/etcd,xueshanf/etcd,dannysauer/etcd,fasaxc/etcd,hroyrh/etcd,jefby/etcd,TheHippo/etcd,freeloop1114/etcd,xiang90/etcd,tempbottle/etcd,shawnps/etcd,dalanlan/etcd,heyitsanthony/etcd,sofuture/etcd,cloudfoundry/etcd,cognusion/etcd,junxu/etcd,bdarnell/etcd,StorReduce/etcd,yichengq/etcd,bluebreezecf/etcd,hpcloud/etcd,jefby/etcd,lumjjb/etcd,lzbgt/etcd,gyuho/etcd,jonboulle/etcd,ktateish/etcd,cchamplin/etcd,mitake/etcd,gtank/etcd,chipironcin/etcd,insequent/etcd,njharman/etcd,achanda/etcd,bdarnell/etcd,crandles/etcd,luxas/etcd,OyTao/etcd_learning,luxas/etcd,bspline/etcd,joshix/etcd,fasaxc/etcd,wowsoso/etcd,SivagnanamCiena/etcd,tguangch/etcd,olark/etcd,junxu/etcd,lzbgt/etcd,snailwalker/etcd,buaazp/etcd,Ulexus/etcd,gityf/etcd,HideoYamauchi/etcd,gae123/etcd,glycerine/etcd,etcd-io/etcd,raoofm/etcd,elimisteve/etcd,a-robinson/etcd,plasticbox/etcd,elimisteve/etcd,zaphod-concur/etcd,sinsharat/etcd,akauppi/etcd,jravetch/etcd,iamqizhao/etcd,jeremyeder/etcd,jeremyeder/etcd,nieanan/etcd,ylong/etcd,vimalk78/etcd,coreos/etcd,ravigadde/etcd,chai2010-playground/etcd,kingland/etcd,Eagle-X/etcd,xdlinear/etcd,nekto0n/etcd,CrazyJvm/etcd,jmptrader/etcd,youxidev/etcd,ravigadde/etcd,purpleidea/etcd,glevand/coreos--etcd,axingblog/etcd,handsomesun/etcd,OyTao/etcd_learning,mqliang/etcd,bakins/etcd,jravetch/etcd,pengzj/etcd,fanminshi/etcd,jchauncey/etcd,raoofm/etcd,hzy001/etcd,arthurcolle/etcd,mike-lerch/etcd,yifan-gu/etcd,hongchaodeng/etcd,snailwalker/etcd,ecnahc515/etcd,axingblog/etcd,heyitsanthony/etcd,jchauncey/etcd,gouyang/etcd,TheHippo/etcd,polvi/etcd,plasticbox/etcd,QuantiumTechnology/etcd,msanterre/etcd,Winslett/etcd,kingland/etcd,tangkun75/etcd,mickep76/etcd,li-ang/etcd,shakefu/etcd,mqliang/etcd,joshix/etcd,xychu/etcd,delkyd/etcd,dannysauer/etcd,sebrandon1/etcd,whuwxl/etcd,gosharplite/etcd,hroyrh/etcd,Ulexus/etcd,swingbach/etcd,sebrandon1/etcd,sinsharat/etcd,zaphod-concur/etcd,Yhgenomics/etcd,svdata/etcd,achanda/etcd,pigfoot/etcd,spotify/etcd,njharman/etcd,oscarrr110/etcd,cvik/etcd,swingbach/etcd,insequent/etcd,handsomesun/etcd,Winslett/etcd,openedbox/etcd,Yhgenomics/etcd,li-ang/etcd,imzhulei/etcd,onlyjob/etcd,mgoelzer/etcd-fork2,HideoYamauchi/etcd,fgbreel/etcd,ecnahc515/etcd,njucslqq/etcd,giantswarm/etcd,hzy001/etcd,xychu/etcd,jmptrader/etcd,mischief/etcd,CrazyJvm/etcd,gyuho/etcd,wowsoso/etcd,armstrongli/etcd,tamird/etcd,cvik/etcd,nieanan/etcd,bakins/etcd,delkyd/etcd,combk8s/etcd,dalanlan/etcd,Ulexus/etcd,cchamplin/etcd,dnaeon/etcd,linuxcer/etcd,oscarrr110/etcd,polvi/etcd,dnaeon/etcd,yyljlyy/etcd,mwitkow/etcd,beni55/etcd,mgoelzer/etcd-fork2,insionng/etcd,rchicoli/etcd,honeyqa/etcd,njucslqq/etcd,improbable-io/etcd,Eagle-X/etcd,xebitstudios/etcd,bluebreezecf/etcd,ajityagaty/etcd,gityf/etcd,YuleiXiao/etcd,qiaochao911/etcd,spotify/etcd,kkaneda/etcd,honeyqa/etcd,spotify/etcd,svdata/etcd,visheshnp/etcd,spacejam/etcd,cockroachdb/etcd,cognusion/etcd,cvik/etcd,QuantiumTechnology/etcd,westhood/etcd