Radius in-place upgrade design document (#87)

Signed-off-by: ytimocin <ytimocin@microsoft.com>

Author: ytimocin

.github/config/en-custom.txt

@@ -1022,3 +1022,26 @@ pdf
 pdfs
 Knative
 genericized
+Istio
+Flux
+reinstallation
+reinstallations
+gapped
+helmclient
+contourclient
+inmemory
+postgresclient
+PVs
+AcquireLock
+postgresql
+preflight
+Preflight
+severities
+PersistentVolumes
+VersionCompatibilityCheck
+ClusterResourceCheck
+ControlPlaneHealthCheck
+CustomConfigValidationCheck
+dir
+rekey
+etcd's