feat: add slack channel (#2920)

Co-authored-by: imbajin <jin@apache.org>

Author: zyxxoo

README.md

@@ -95,6 +95,7 @@ and we are looking forward to working with you to build an excellent open-source
 
 - [GitHub Issues](https://github.com/apache/hugegraph/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://github.com/apache/hugegraph-doc/blob/master/assets/images/wechat.png?raw=true" alt="QR png" width="300"/>