Add clusterclass-related documentation

Signed-off-by: Vui Lam <vui@vmware.com>

docs/cli/clusterclass/README.md

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+# ClusterClass Support
+
+Documentation for cluster LCM using ClusterClass.
+
+## Table of Contents
+
+* [ClusterClass Support](cluster-class-support.md)
+* ClusterClass Developer Overview (To be added)

docs/cli/clusterclass/cluster-class-support.md

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+# Support for ClusterClass-based LCM
+
+## Intro
+
+The ClusterClass functionality in Cluster API supports the server-side
+management CAPI cluster topologies. By leveraging this functionality
+not only will we be able to encapsulate common cluster configuration
+scenarios at the server end in a reusable fashion, we will also be able
+eliminate a significant fraction of client-side processing required to create a
+workload cluster today.
+
+## Approach
+
+To introduce ClusterClass in a backward-compatible fashion, all the existing
+ytt-base client-side templates are retained. The CLI will accept the legacy
+cluster configuratiion file (based on flat key-value pairs) as well as a CAPI
+ClusterClass-based Cluster resource as appropriate for the target
+infrastructure. The latter is sometimes referred to as the 'CCluster.yaml'
+
+Where possible, legacy configuration will be translated into a CCluster.yaml,
+and users will be urged to use the new format where possible in the future.
+
+Existing ytt overlays are retained. New files to support ClusterClass
+are introduced in a non-overlapping fashion as separate directories (name
+'yttcc' and 'cconly') across all supported target infrastructures.
+
+yttcc/ : contains overlays required to translate legacy config values into
+CCluster.yaml. They will be included in an upcoming version of the CLI which
+supports ClusterClass-based cluster creation.
+
+cconly/ : contains overlays required configure the ClusterClass and associated
+templates themselves. They are included for development purposes for now, and
+will eventually not be installed on the CLI host.
+
+The reason for the above is that ClusterClass's and their associate templates
+are being assembled as Carvel packages and deployed to the bootstrap cluster
+and management cluster through these packages (to enable ClusterClass-based
+creation of the management cluster and workload clusters respectively). One can
+assume that there will always be a default set of ClusterClass and templates
+on each Management Cluster deployed.
+
+Note: To enable the deployment of ClusterClass package on the bootstrap cluster,
+kapp-controller is now installed into the bootstrap cluster as well.