doc: add note to described the possibility to use MultiNamespacedCach… (#2281)

Author: camilamacedo86

doc/user-guide.md

@@ -63,6 +63,17 @@ By default this will be the namespace that the operator is running in. To watch
 mgr, err := manager.New(cfg, manager.Options{Namespace: ""})
 ```
 
+It is also possible to use the [MultiNamespacedCacheBuilder][multi-namespaced-cache-builder] to watch a specific set of namespaces:
+```Go
+var namespaces []string // List of Namespaces
+// Create a new Cmd to provide shared dependencies and start components
+mgr, err := manager.New(cfg, manager.Options{
+   NewCache: cache.MultiNamespacedCacheBuilder(namespaces),
+   MapperProvider:     restmapper.NewDynamicRESTMapper,
+   MetricsBindAddress: fmt.Sprintf("%s:%d", metricsHost, metricsPort),
+})
+```
+
 By default the main program will set the manager's namespace using the value of `WATCH_NAMESPACE` env defined in `deploy/operator.yaml`.
 
 ## Add a new Custom Resource Definition
@@ -668,3 +679,4 @@ When the operator is not running in a cluster, the Manager will return an error
 [result_go_doc]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/reconcile#Result
 [metrics_doc]: ./user/metrics/README.md
 [quay_link]: https://quay.io
+[multi-namespaced-cache-builder]: https://godoc.org/github.com/kubernetes-sigs/controller-runtime/pkg/cache#MultiNamespacedCacheBuilder