Documentation: add etcd-rest to libraries-and-tools.md
Documentation: add etcd-rest to libraries-and-tools.md

Add link to the etcd-rest client under the 'Java libraries' sub-section.

Fixes #4906

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