values.yaml: Make it more clear how to utilize connectInject.namespaceSelector

[david-yu]

Describe that connectInject.default should be used if trying to enable/disable connectInject by default. Addresses: #698

Changes proposed in this PR:

• Comment to describe you should use namespaceSelector with `connectInject.default

How I've tested this PR:

❯ cat config.yaml
global:
  name: consul
  datacenter: dc1
  tls:
    enabled: true
server:
  replicas: 1
ui:
  enabled: true
meshGateway:
  enabled: true
  replicas: 1
connectInject:
  enabled: true
  default: true
  namespaceSelector: |
    matchLabels:
      connect-inject : enabled
controller:
  enabled: true

❯ kubectl describe ns foo
Name:         foo
Labels:       connect-inject=enabled
Annotations:  <none>
Status:       Active

Resource Quotas
 Name:                              gke-resource-quotas
 Resource                           Used  Hard
 --------                           ---   ---
 count/ingresses.extensions         0     100
 count/ingresses.networking.k8s.io  0     100
 count/jobs.batch                   0     5k
 pods                               2     1500
 services                           2     500

No LimitRange resource.

❯ kubectl get pods -n foo
NAME                        READY   STATUS    RESTARTS   AG
tcp-echo-866d7f8dcb-wxqnt   2/2     Running   0          70s

❯ kubectl get pods -n bar
NAME                        READY   STATUS    RESTARTS   AGE
tcp-echo-866d7f8dcb-n7j8s   1/1     Running   0          59s

How I expect reviewers to test this PR:

Checklist:

• Tests added
• CHANGELOG entry added (HashiCorp engineers only, community PRs should not add a changelog entry)