Add README.md file for fluentd-elasticsearch image directory

contrib/logging/fluentd-es-image/README.md

+# Collecting Docker Log Files with Fluentd and Elasticsearch
+This directory contains the source files needed to make a Docker image
+that collects Docker container log files using [Fluentd](http://www.fluentd.org/)
+and sends them to an instance of [Elasticsearch](http://www.elasticsearch.org/).
+This image is designed to be used as part of the [Kubernetes](https://github.com/GoogleCloudPlatform/kubernetes)
+cluster bring up process. The image resides at DockerHub under the name
+[kubernetes/fluentd-eslasticsearch](https://registry.hub.docker.com/u/kubernetes/fluentd-elasticsearch/).