feat: add slack channel (#441)

* feat: add slack channel

* Update README.md

---------

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

Author: zyxxoo

README.md

@@ -74,6 +74,7 @@ The functions of this system include but are not limited to:
 - [GitHub Issues](https://github.com/apache/incubator-hugegraph-doc/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)
 - Security Email: [security@hugegraph.apache.org](mailto:security@hugegraph.apache.org) (Report SEC problems)
+- 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="./assets/images/wechat.png" alt="QR png" width="350"/>