Document sig-multicluster decision about reserved namespaces.

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+# Multicluster reserved namespaces
+
+@perotinus
+
+06/06/2018
+
+## Background
+
+sig-multicluster has identified the need for a canonical set of namespaces that
+can be used for supporting multicluster applications and use cases. Initially,
+an [issue](https://github.com/kubernetes/cluster-registry/issues/221) was filed
+in the cluster-registry repository describing the need for a namespace that
+would be used for public, global cluster records. This topic was further
+discussed at the
+[SIG meeting on June 5, 2018](https://www.youtube.com/watch?v=j6tHK8_mWz8&t=3012)
+and in a
+[thread](https://groups.google.com/forum/#!topic/kubernetes-sig-multicluster/8u-li_ZJpDI)
+on the SIG mailing list.
+
+## Reserved namespaces
+
+We determined that there is currently a strong case for two reserved namespaces
+for multicluster use:
+
+-   `kube-multicluster-public`: a global, public namespace for storing cluster
+    registry Cluster objects. If there are other custom resources that
+    correspond with the global, public Cluster objects, they can also be stored
+    here. For example, a custom resource that contains cloud-provider-specific
+    metadata about a cluster. Tools built against the cluster registry can
+    expect to find the canonical set of Cluster objects in this namespace[1].
+
+-   `kube-multicluster-system`: an administrator-accessible namespace that
+    contains components, such as multicluster controllers and their
+    dependencies, that are not meant to be seen by most users directly.
+
+The definition of these namespaces is not intended to be exhaustive: in the
+future, there may be reason to define more multicluster namespaces, and
+potentially conventions for namespaces that are replicated between clusters (for
+example, to support a global cluster list that is replicated to all clusters
+that are contained in the list).
+
+## Conventions for reserved namesp[aces
+
+By convention, resources in these namespaces are local to the clusters in which
+they exist and will not be replicated to other clusters.
+
+[1] Tools are by no means compelled to look in this namespace for clusters, and
+can choose to reference Cluster objects from other namespaces as is suitable to
+their design and environment.