README.md - missing kubeconfig reference

[Lewis Denham-Parry]

First of all, LOVE Hashicraft!!!

Following through the README, the kubectl exec command doesn't work due to the nested shell not having the downloaded kubeconfig file, this causes a potential error in connecting to whatever cluster is set with the default config.

Example fix below:

KUBECONFIG=$PWD/kubeconfig.yaml kubectl exec -it \
  $(KUBECONFIG=$PWD/kubeconfig.yaml kubectl get pods -l app=minecraft -o=jsonpath='{.items[0].metadata.name}') \
  bash