doc: add backword_compatibility.md
doc: add backword_compatibility.md

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