message: readme: add brief introduction for nydus-overlayfs
  An introduction to `nydus-overlyfs` helper was lacked.
  In addition, let's move harbor acceleration service introduction
  to a seperated section. Keep the table only to tools belonging to
  this project repo.
diff: @@ -35,7 +35,7 @@ Currently Nydus includes following tools:
  | nydusctl | Nydusd CLI client, query daemon's working status/metrics and configure it |
  | ctr-remote | An enhanced `containerd` CLI tool enable nydus support with `containerd` ctr |
  | nydus-docker-graphdriver | Works as a `docker` remote graph driver to control how images and containers are stored and managed |
  -| [acceleration-service](https://github.com/goharbor/acceleration-service) | Provides a general service for Harbor to support image acceleration based on kinds of accelerator like Nydus and eStargz etc. |
  +| nydus-overlayfs | `Containerd` mount helper to invoke overlayfs mount with tweaking mount options a bit. So nydus prerequisites can be passed to vm-based runtime |
  To try nydus image service:
  @@ -76,6 +76,10 @@ Normally, users do not need to start `nydusd` by hand. It is started by `nydus-s
  Run Nydusd Daemon to serve Nydus image: [Nydusd](./docs/nydusd.md).
  +## Build Images via Harbor
  +
  +Nydus cooperates with Harbor community to develop [acceleration-service](https://github.com/goharbor/acceleration-service) which provides a general service for Harbor to support image acceleration based on kinds of accelerators like Nydus, eStargz, etc.
  +
  ## Docker graph driver support
  Docker graph driver is also accompanied, it helps to start container from nydus image. For more particular instructions, please refer to
  