Doc: add learner in runtime-configuration.md
Doc: add learner in runtime-configuration.md

ajityagaty/etcd,mqliang/etcd,rnaveiras/etcd,mkumatag/etcd,coreos/etcd,li-ang/etcd,mqliang/etcd,dannysauer/etcd,mqliang/etcd,li-ang/etcd,gyuho/etcd,rnaveiras/etcd,cockroachdb/etcd,mkumatag/etcd,etcd-io/etcd,etcd-io/etcd,coreos/etcd,cockroachdb/etcd,mkumatag/etcd,rnaveiras/etcd,etcd-io/etcd,ajityagaty/etcd,ajityagaty/etcd,gyuho/etcd,dannysauer/etcd