message: nydus-test: add the missing parameter 'artifacts'
  Add the missing parameter 'artifacts' for anchor_conf.json in README.md,
  which will cause an error when generating rootfs. KeyError: 'artifacts'
diff: @@ -47,6 +47,9 @@ Nydus-test is controlled and configured by `anchor_conf.json`. Nydus-test will t
  "busybox:latest"
  ]
  },
  + "artifacts": {
  + "containerd": "/usr/bin/containerd"
  + },
  "logging_file": "stderr",
  "target": "gnu"
  }
  