Please add this to documentation - minLength with ObservableArray

For others trying to solve this problem, which is rather easy, but not documented (if wrong let me know, seems most examples just deal with observable), I thought I'd share:

ie. property to validate that is observableArray

var items.observableArray().extend({
minLength:{
message: "At least one item is required"
}
});

Then in the UI to add the message:
ul data-bind="foreach: items()"
...

(I'm using twitter bootstrap...)
span class="alert alert-error" data-bind="validationMessage: items"></span