Add readme for fluentd-gcp addon

cluster/addons/fluentd-gcp/README.md

+# Stackdriver Logging Agent
+==============
+
+Stackdriver Logging Agent is a DaemonSet which spawns a pod on each node
+that reads logs, generated by kubelet, container engine and containers
+and sends them to the Stackdriver. When logs are exported to the Stackdriver,
+they can be searched, viewed, and analyzed.
+
+Learn more at: https://kubernetes.io/docs/tasks/debug-application-cluster/logging-stackdriver
+
+[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/cluster/addons/fluentd-gcp/README.md?pixel)]()