Doc update to reflect minimum cluster requirements

[charandas]

@brndnmtthws Saw your comment here.

Thought it might serve some of us newbies to have this in the readme.

[charandas]

Or actually, this may not be needed, if one is running non critical applications behind the load balancer. In that case, one can specify:

"upgradeStrategy": {
  "minimumHealthCapacity": 0
}

[brndnmtthws]

Thanks!