Add note in RichTextEditor documentation about asRequired use

articles/components/rich-text-editor/index.adoc

@@ -156,6 +156,22 @@ For the Flow component, the default is the HTML format, which is also used autom
 
 To read, write, or bind the component's value using the Delta format, use the https://vaadin.com/api/platform/{moduleMavenVersion:com.vaadin:vaadin}/com/vaadin/flow/component/richtexteditor/RichTextEditor.html#asDelta()[`RichTextEditor.asDelta()`] wrapper.
 
+[NOTE]
+Due nature of the component the default `asRequired` validator is not working as expected with `Binder`. Hence custom validator needs to be used.
+
+[.example]
+
+--
+
+ifdef::flow[]
+[source,java]
+----
+binder.forField(richTextEditor)
+    .asRequired(Validator.from(value -> !richTextEditor.isEmpty(), "Required"))
+----
+endif::[]
+--
+
 For the web component, to read or write the value in the Delta format, use the `value` property.
 
 To read or write the value in the HTML format, use the `htmlValue` property and the `dangerouslySetHtmlValue` method.