New channel for security assessment of cluster api

From SIG-Security's last weekly meeting an action item was to create a new channel in order to facilitate a focussed discussion on security assessment of cluster-api (a sub project of kubernetes). This is a first attempt of a security assessment of a sub-project performed in collaboration with sig-security of the parent graduated project. 

Since we are breaking new ground here, I am happy to revisit if this channel should be a prefixed as `wg` or `sig`.  We intend to periodically revisit the results from our assessments, so I do not see this as a time-bound effort but more of a continuous improvement cycle.

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

@@ -2,3 +2,4 @@ channels:
   - name: sig-security
   - name: sig-security-docs
   - name: sig-security-tooling
+  - name: sig-security-assess-capi