docs: instructions for taking over cluster CR with helm chart
docs: instructions for taking over cluster CR with helm chart

The CephCluster CR for an existing cluster can be taken over by
the new cluster helm chart if desired, for simpler management
of the RBAC and other resources.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

rook/rook,rook/nfs,rook/nfs,rook/rook,rook/nfs,rook/rook,rook/rook,rook/rook