Add to README about nsenter and udp
Add to README about nsenter and udp

* nsenter will give you a way of accessing the minecraft server
  to send server commands for example. #2

* udp is required to enable the query protocol

erubboli/docker-minecraft,matthewbaggett/docker-minecraft,kroonwijk/rpi-minecraft,zebMcCorkle/docker-minecraft,jaryn/docker-minecraft,spitimage/docker-scriptcraft,idxbroker/docker-minecraft,harborspace/docker-minecraft,overshard/docker-minecraft