slack-config: add #krew
slack-config: add #krew

https://sigs.k8s.io/krew is a vibrant SIG CLI project with user and developer
communities.

Proposing `#krew` channel to Kubernetes Slack for colalboration.

Signed-off-by: Ahmet Alp Balkan <c786c8527fd6be7431bb94ca48b6f76cc06787cc@google.com>

kubernetes/community,pwittrock/community,kubernetes/community,kubernetes/community,pwittrock/community