message: Readme: update slack url to one that never expires
  Also this patch formatted Readme.md by a formatter.
diff: @@ -10,22 +10,22 @@ The following benchmarking result shows the performance improvement compared wit
  Nydus' key features include:
  -* Container images are downloaded on demand
  -* Chunk level data deduplication
  -* Flatten image metadata and data to remove all intermediate layers
  -* Only usable image data is saved when building a container image
  -* Only usable image data is downloaded when running a container
  -* End-to-end image data integrity
  -* Compatible with the OCI artifacts spec and distribution spec
  -* Integrated with existing CNCF project Dragonfly to support image distribution in large clusters
  -* Different container image storage backends are supported
  +- Container images are downloaded on demand
  +- Chunk level data deduplication
  +- Flatten image metadata and data to remove all intermediate layers
  +- Only usable image data is saved when building a container image
  +- Only usable image data is downloaded when running a container
  +- End-to-end image data integrity
  +- Compatible with the OCI artifacts spec and distribution spec
  +- Integrated with existing CNCF project Dragonfly to support image distribution in large clusters
  +- Different container image storage backends are supported
  Currently the repository includes following tools:
  -* A `nydusify` tool to convert an OCI format container image into a nydus format container image
  -* A `containerd-nydus-grpc` daemon to serve as containerd remote snapshotter and setup container rootfs with nydus
  -* A `nydus-image` tool to convert an unpacked container image into a nydus format image
  -* A `nydusd` daemon to parse a nydus format image and expose a FUSE mountpoint for containers to access
  +- A `nydusify` tool to convert an OCI format container image into a nydus format container image
  +- A `containerd-nydus-grpc` daemon to serve as containerd remote snapshotter and setup container rootfs with nydus
  +- A `nydus-image` tool to convert an unpacked container image into a nydus format image
  +- A `nydusd` daemon to parse a nydus format image and expose a FUSE mountpoint for containers to access
  ## Build Binary
  @@ -67,14 +67,14 @@ You can reach the community via Dingtalk and Slack
  Any bug report, feature requirement, and technique discussion and cooperation are welcomed and expected!
  -* Slack
  +- Slack
  - Join our Slack [workspace](https://join.slack.com/t/nydusimageservice/shared_invite/zt-o67n0ye9-HuN0c2MPzjUVJvF~W4PnXg)
  + Join our Slack [workspace](https://join.slack.com/t/nydusimageservice/shared_invite/zt-pz4qvl4y-WIh4itPNILGhPS8JqdFm_w)
  -* Dingtalk
  +- Dingtalk
  Join nydus-devel group by clicking [URL](https://qr.dingtalk.com/action/joingroup?code=v1,k1,YfGzhaTOnpm10Bf+/ohz4WcuDEIe9nTIjo+MPuIgRGQ=&_dt_no_comment=1&origin=11) from your phone.
  - You can also search our talking group by number *34971767* and QR code
  + You can also search our talking group by number _34971767_ and QR code
  <img src="./misc/dingtalk.jpg" width="250" height="300"/>
  