update cli doc

Signed-off-by: jesus m. rodriguez <jesusr@redhat.com>

website/content/en/docs/cli/operator-sdk_bundle_validate.md

@@ -81,13 +81,18 @@ To validate a bundle against the validator for Good Practices specifically, in a
 
 To validate a bundle against the (alpha) validator for Deprecated APIs specifically, in addition to required bundle validators:
 
-  $ operator-sdk bundle validate ./bundle --select-optional name=alpha-deprecated-apis --optional-values=k8s-version=1.22	
+  $ operator-sdk bundle validate ./bundle --select-optional name=alpha-deprecated-apis --optional-values=k8s-version=1.22
+
+To validate a bundle against an external validator, in addition to required bundle validators:
+
+  $ operator-sdk bundle validate ./bundle --alpha-select-external /path/to/external-validator[:/path/to/optional-second-validator]
 	
 ```
 
 ### Options
 
 ```
+      --alpha-select-external string                         Selector to select external validators to run. It should be set to a Unix path list ("/path/to/e1.sh:/path/to/e2")
   -h, --help                                                 help for validate
   -b, --image-builder string                                 Tool to pull and unpack bundle images. Only used when validating a bundle image. One of: [docker, podman, none] (default "docker")
       --list-optional                                        List all optional validators available. When set, no validators will be run