doc/security.md: add notes for proxy security
doc/security.md: add notes for proxy security

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