adding documentation

Author: Daniel Feinberg

doc/user-guide.md

@@ -65,11 +65,21 @@ func main() {
 }
 ```
 
-**Note:** The number of concurrent informer workers can be configured with an additional Watch option. The default value is 1 if an argument is not given.
+#### Options
+**Worker Count**
+The number of concurrent informer workers can be configured with an additional Watch option. The default value is 1 if an argument is not given.
 ```Go
 sdk.Watch("cache.example.com/v1alpha1", "Memcached", "default", 5, sdk.WithNumWorkers(n))
 ```
 
+**Label Selector**
+Label selectors allow the watch to filter resources by kubernetes labels. It can be sepecified using the standard kubernetes label selector format:
+[https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors]
+
+```Go
+sdk.Watch("cache.example.com/v1alpha1", "Memcached", "default", 5, sdk.WithLabelSelector("app=myapp"))
+```
+
 ### Define the Memcached spec and status
 
 Modify the spec and status of the `Memcached` CR at `pkg/apis/cache/v1alpha1/types.go`: