Document init containers and migrations

[sobolevn]

That's how we run migrate and similar scripts in real-life:

initContainers:
  - args:
      - ...
    command:
      - ...
    envFrom:
      - configMapRef:
        name: django-env-map
    image:
    imagePullPolicy: Always
    name: django_bundle
    volumeMounts:
      - mountPath: /var/www/django
        name: ...
      - mountPath: /code/locale
        name: ...