readme update for fluentd-gcp-scaler

cluster/addons/fluentd-gcp/README.md

@@ -8,4 +8,67 @@ they can be searched, viewed, and analyzed.
 
 Learn more at: https://kubernetes.io/docs/tasks/debug-application-cluster/logging-stackdriver
 
+## Troubleshooting
+
+In Kubernetes clusters in version 1.10.0 or later, fluentd-gcp DaemonSet can be
+manually scaled. This is useful e.g. when the applications running in the
+cluster are sending a large volume of logs (i.e. over 100kB/s), causing
+fluentd-gcp to fail with OOM errors. Conversely, if the applications aren't
+generating a lot of logs, it may be useful to reduce the amount of resources
+consumed by fluentd-gcp, making them available to other applications. To learn
+more about Kubernetes resource requests and limits, see the official
+documentation ([CPU][cpu], [memory][memory]). The amount of resources requested
+by fluentd-gcp on every node in the cluster can be fetched by running following
+command:
+
+```
+$ kubectl get ds -n kube-system -l k8s-app=fluentd-gcp -o custom-columns=NAME:.metadata.name,CPU_REQUEST:.spec.template.spec.containers[].resources.requests.cpu,MEMORY_REQUEST:.spec.template.spec.containers[].resources.requests.memory,MEMORY_LIMIT:.spec.template.spec.containers[].resources.limits.memory
+```
+
+This will display an output similar to the following:
+
+```
+NAME                  CPU_REQUEST   MEMORY_REQUEST   MEMORY_LIMIT
+fluentd-gcp-v2.0.15   100m          200Mi            300Mi
+```
+
+In order to change those values, a [ScalingPolicy][scalingPolicy] needs to be
+defined. Currently, only base values are supported (no automatic scaling). The
+ScalingPolicy can be created using kubectl. E.g. to set cpu request to 101m,
+memory request to 150Mi and memory limit to 400Mi:
+
+```
+$ cat <<EOF | kubectl apply -f -
+apiVersion: scalingpolicy.kope.io/v1alpha1
+kind: ScalingPolicy
+metadata:
+  name: fluentd-gcp-scaling-policy
+  namespace: kube-system
+spec:
+  containers:
+  - name: fluentd-gcp
+    resources:
+      requests:
+      - resource: cpu
+        base: 101m
+      - resource: memory
+        base: 150Mi
+      limits:
+      - resource: memory
+        base: 400Mi
+EOF
+```
+
+To remove the override and go back to GKE-provided defaults, it is enough to
+just remove the ScalingPolicy:
+
+```
+$ kubectl delete -n kube-system scalingpolicies.scalingpolicy.kope.io/fluentd-gcp-scaling-policy
+```
+
+[cpu]: https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-resource/
+[memory]: https://kubernetes.io/docs/tasks/configure-pod-container/assign-memory-resource/
+[scalingPolicy]: https://github.com/justinsb/scaler
+
+
 [![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/cluster/addons/fluentd-gcp/README.md?pixel)]()