CONTRIBUTING: mention the Slack channel and office hours calls

The README did mention our Slack workspace,
but it's helpful to make it clear that we have a #contributing channel.

Similarly, the README already mentions our community channel,
but it's good to point out we hold regular contributor office hours.

This document is starting to get quite long,
but both of these bits of information feel like a must have for now.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I8f8a3d4afa3a482095319a7d18fcf890b4c4a249
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1172993
Reviewed-by: Paul Jolly <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

CONTRIBUTING.md

@@ -64,6 +64,11 @@ inherently distributed nature of open-source this coordination is important
 because it very often serves as the main form of communication between
 contributors.
 
+You can also exchange ideas or feedback with other contributors via the
+`#contributing` [Slack channel](https://cuelang.slack.com/archives/CMY132JKY),
+as well as the contributor office hours calls which we hold via the
+[community calendar](https://cuelang.org/s/community-calendar) once per week.
+
 ### Check the issue tracker
 
 Whether you already know what contribution to make, or you are searching for an