Per coreos/prometheus-operator#2519

Fix go get jsonnet command.

Changed kubectl apply -f to kubectl create -f. kubectl apply command leads to conflicts and race conditions. kubectl create works the first time you run it

Resources can be created _and_ updated with `apply` without receiving errors

Currently there is no documentation on the Pod Anti-Affinity mixin which makes it
difficult to find. To address this, this PR adds a section to the README on how one
can use it.

contrib/kube-prometheus: Add section on Prometheus Adapter requirements to README.md