Update config.yaml

Adding a channel for the etcd self assessment

Author: aladewberry

communication/slack-config/sig-security/config.yaml

@@ -4,4 +4,5 @@ channels:
   - name: sig-security-docs
   - name: sig-security-tooling
   - name: sig-security-assessments
+  - name: sig-security-assess-etcd
   - name: sig-security-assess-vsphere-csi-driver