Cockroach errors out when using said command:

```shell
▶  kubectl run -it --rm cockroach-client --image=cockroachdb/cockroach --restart=Never --command -- ./cockroach sql --host cockroachdb-public
Waiting for pod default/cockroach-client to be running, status is Pending, pod ready: false
Waiting for pod default/cockroach-client to be running, status is Pending, pod ready: false
Waiting for pod default/cockroach-client to be running, status is Pending, pod ready: false
If you don't see a command prompt, try pressing enter.
                                                      Error attaching, falling back to logs: unable to upgrade connection: container cockroach-client not found in pod cockroach-client_default
Error: problem using security settings, did you mean to use --insecure?: problem with CA certificate: not found
Failed running "sql"
Waiting for pod default/cockroach-client to terminate, status is Running
pod "cockroach-client" deleted
```

This PR updates the README.md to include --insecure in the client command