README.md: remove a dead link to chat.opencontainers.org

https://chat.opencontainers.org/ is down (NXDOMAIN)

See opencontainers/.github issue 39

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

README.md

@@ -79,7 +79,8 @@ You can subscribe and join the mailing list on [Google Groups](https://groups.go
 
 OCI discussion happens in the following chat rooms, which are all bridged together:
 
-- #general channel on [OCI Slack](https://chat.opencontainers.org/)
+<!-- https://chat.opencontainers.org is currently down: https://github.com/opencontainers/.github/issues/39 -->
+- #general channel on [OCI Slack](https://join.slack.com/t/opencontainers/shared_invite/zt-vzxfzkkl-CbsYY9zwMhSTkXlvvhmBVA)
 - #opencontainers:matrix.org
 - #opencontainers on freenode.net