Add docs for KubeSchedulerConfiguration

Add some example docs for addon objects / custom KubeSchedulerConfiguration.

docs/addon_objects.md

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+# Passing additional configuration objects
+
+kOps has initial support for passing additional objects to the cluster, and recognizes a few "well known" objects.
+
+This support is currently gated behind the `ClusterAddons` feature-flag (i.e. `export KOPS_FEATURE_FLAGS=ClusterAddons`)
+
+Objects that are not well-known will be applied to the cluster.  Well-known objects will have special handling.
+
+# Well-Known Objects
+
+## KubeSchedulerConfiguration (group: kubescheduler.config.k8s.io)
+
+KubeSchedulerConfiguration objects allow for custom configuration of
+kube-scheduler, the component responsible for assigning Pods to Nodes.
+
+Special handling:  the configuration will be written to a file on control plane nodes,
+and the kube-scheduler component will be configured to read from that file.
+
+Example usage:
+```
+export KOPS_FEATURE_FLAGS=ClusterAddons
+kops create cluster --name=kubescheduler.k8s.local --zones us-east-2a --add docs/examples/addons/kubescheduler.yaml
+kops update cluster --name=kubescheduler.k8s.local --yes --admin
+kops validate cluster --name=kubescheduler.k8s.local
+```

docs/examples/addons/kubescheduler.yaml

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+# This is an example "addon" object to provide additional/custom kube-scheduler configuration
+apiVersion: kubescheduler.config.k8s.io/v1beta2
+kind: KubeSchedulerConfiguration
+clientConnection:
+  burst: 100
+  qps: 3.1