CONTRIBUTING: clarify commit message style
CONTRIBUTING: clarify commit message style

Signed-off-by: Sam Batschelet <eebdbe215779e06aa0285d26936ee9141aa3d260@redhat.com>

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