Add multi-network channel.

Author: mskrocki

communication/slack-config/channels.yaml

@@ -397,6 +397,7 @@ channels:
   - name: sig-instrumentation
   - name: sig-multicluster
   - name: sig-network
+  - name: sig-network-multi-network
   - name: sig-node
   - name: sig-node-bug-scrub
   - name: sig-node-rkt