Adds kn nightly and latest release image URL in docs (#466)
Adds kn nightly and latest release image URL in docs (#466)

* Nightly: `gcr.io/knative-nightly/knative.dev/client/cmd/kn`
 * Latest release: `gcr.io/knative-releases/knative.dev/client/cmd/kn`
knative/client,knative/client