README.md: Add link to CNCF slack

I recently added this because there's a need for a development-focused
sync chat instance and the CNCF Slack makes sense alongside
other CNCF projects.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

README.md

@@ -34,7 +34,8 @@ start by looking at [ADOPTERS.md](ADOPTERS.md).
 
 ## Community discussion
 
-The [Github discussion forum](https://github.com/containers/bootc/discussions) is enabled.
+- [Github discussion forum](https://github.com/containers/bootc/discussions) for async discussion
+- [#bootc-dev on CNCF Slack](https://cloud-native.slack.com/archives/C08SKSQKG1L) for live chat
 
 This project is also tightly related to the previously mentioned Fedora/CentOS bootc project,
 and many developers monitor the relevant discussion forums there. In particular there's a