Adding sig-security-tooling slack channel

Author: ericsmalling

communication/slack-config/channels.yaml

@@ -333,6 +333,7 @@ channels:
   - name: sig-scalability
   - name: sig-scheduling
   - name: sig-security
+  - name: sig-security-tooling
   - name: sig-service-catalog
   - name: sig-testing
   - name: sig-ui