Slack channel for discussion of the PureLB load balancer

[PureLB](https://gitlab.com/purelb/purelb) is a new open source load
balancer controller for Kubernetes.  May we please have a slack
channel for user discussion?

Thank you

communication/slack-config/channels.yaml

@@ -287,6 +287,7 @@ channels:
   - name: prow
   - name: prow-alerts
   - name: pt_br-users
+  - name: purelb-users
   - name: raksh
   - name: rancher
   - name: random