Add rule about not extending Data.define

See also:
* [PR adding new cop for this rule](rubocop/rubocop#11728)
* [Original issue](rubocop/rubocop#11696)

README.adoc

@@ -5842,6 +5842,21 @@ config
 
 This is simpler and more efficient.
 
+=== Don't Extend `Data.define` [[no-extend-data-define]]
+
+Don't extend an instance initialized by `Data.define`.
+Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times.
+
+[source,ruby]
+----
+# bad
+class Person < Data.define(:first_name, :last_name)
+end
+
+# good
+Person = Data.define(:first_name, :last_name)
+----
+
 == Tools
 
 Here are some tools to help you automatically check Ruby code against this guide.