Add NEWS entry for [[Feature #20429]]

Followup: #10532

Author: byroot

NEWS.md

@@ -122,6 +122,11 @@ See GitHub releases like [GitHub Releases of Logger](https://github.com/ruby/log
   a warning on verbose mode (`-w`).
   [[Feature #15554]]
 
+* Redefining some core methods that are specially optimized by the interpeter
+  and JIT like `String.freeze` or `Integer#+` now emits a performance class
+  warning (`-W:performance` or `Warning[:performance] = true`).
+  [[Feature #20429]]
+
 [Feature #13557]: https://bugs.ruby-lang.org/issues/13557
 [Feature #15554]: https://bugs.ruby-lang.org/issues/15554
 [Feature #16495]: https://bugs.ruby-lang.org/issues/16495