docs(upgrade): Add upgrade troubleshooting guide (openservicemesh#2377)

Signed-off-by: Kalya Subramanian <kasubra@microsoft.com>

docs/troubleshooting/upgrade.md

@@ -0,0 +1,17 @@
+# OSM Upgrade Troubleshooting Guide
+
+## Server could not find requested resource
+If the [upgrade CRD guide](../upgrade_guide.md##crd-upgrades) was not followed, it is possible that the installed CRDs are out of sync with the OSM controller.
+
+The OSM controller will then crash with errors similar to this:
+```
+reflector.go:178] pkg/mod/k8s.io/client-go@v0.18.6/tools/cache/reflector.go:125: Failed to list *v1alpha2.TrafficTarget: the server could not find the requested resource (get traffictargets.access.smi-spec.io)
+```
+To resolve these errors:
+1. Checkout the correct release branch of the [repo](https://github.com/openservicemesh/osm) and run the following commands from the root. 
+1. Delete existing CRDs and Custom Resources (TrafficTargets, TrafficSplits, etc.)
+   - `./scripts/cleanup/crd-cleanup.sh`
+1. Install the new CRDs
+   - `kubectl apply -f charts/osm/crds/`
+1. Restart the osm-controller pod
+1. Recreate CustomResources