README: discussion to slack and kubernetes-dev
README: discussion to slack and kubernetes-dev

I think that IRC channel is deprecated, move people to slack.
AlmogBaku/heapster,burmanm/heapster,Stackdriver/heapster,huangyuqi/heapster,AlmogBaku/heapster,adfin/statster,krzyzacy/heapster,burmanm/heapster,Stackdriver/heapster,huangyuqi/heapster,johanneswuerbach/heapster,bluebreezecf/heapster,andyxning/heapster,andyxning/heapster,johanneswuerbach/heapster,luxas/heapster,bluebreezecf/heapster,bruceauyeung/heapster,adfin/statster,piosz/heapster,piosz/heapster,krzyzacy/heapster,bruceauyeung/heapster,whuwxl/heapster,luxas/heapster,whuwxl/heapster