Documentation: add a doc for using the go client library
Documentation: add a doc for using the go client library

This commit adds a document that provides tips of how to use the go
client library. Currently it describes how to use the
client.SelectionMode parameter that is added in
https://github.com/coreos/etcd/pull/4030.

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