feat: add slack channel (#702)

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Co-authored-by: imbajin <jin@apache.org>
Co-authored-by: VGalaxies <dyc1904821183@gmail.com>

Author: 3 people

README.md

@@ -74,6 +74,7 @@ hugegraph-toolchain is licensed under [Apache 2.0](https://github.com/apache/hug
 
  - [GitHub Issues](https://github.com/apache/hugegraph-toolchain/issues): Feedback on usage issues and functional requirements (quick response)
  - Feedback Email: [dev@hugegraph.apache.org](mailto:dev@hugegraph.apache.org) ([subscriber](https://hugegraph.apache.org/docs/contribution-guidelines/subscribe/) only)
+ - Slack: [ASF Online Channel](https://the-asf.slack.com/archives/C059UU2FJ23)
  - WeChat public account: Apache HugeGraph, welcome to scan this QR code to follow us.
 
  <img src="https://raw.githubusercontent.com/apache/hugegraph-doc/master/assets/images/wechat.png" alt="QR png" width="350"/>