Document Security Group behaviour

[gargana]

Document Security group behaviour for additional node groups.

• By default if no Security Group(SG) is provided we add the nodegroup to the existing default nodegroup SG.
• If you provide your own SG (if you want to strictly control access)
  • Remember to allow for core DNS queries to default SG (Add an IngressRule to that SG)
  • Allow access to/from the Control Plane SG.