Added strategy update instructions to docs

docs/book/src/tasks/experimental-features/cluster-resource-set.md

@@ -8,3 +8,8 @@ The `ClusterResourceSet` feature is introduced to provide a way to automatically
 
 More details on `ClusterResourceSet` and an example to test it can be found at:
 [ClusterResourceSet CAEP](https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20200220-cluster-resource-set.md)
+
+## Update from `ApplyOnce` to `Reconcile`
+
+The `strategy` field is immutable so existing CRS can't be updated directly. However, CAPI won't delete the managed resources in the target cluster when the CRS is deleted.
+So if you want to start using the `Reconcile` strategy, delete your existing CRS and create it again with the updated `strategy`.