Added entry about ext/readline

NEWS.md

@@ -82,6 +82,11 @@ changelog for details of the default gems or bundled gems.
 
 ## Stdlib compatibility issues
 
+### `ext/readline` is retired
+
+* We have `reline` that is pure Ruby implementation compatible with `ext/readline` API. We rely on `reline` in the future. If you need to use `ext/readline`, you can install `ext/readline` via rubygems.org with `gem install readline-ext`.
+* We no longer need to install libraries like `libreadline` or `libedit`.
+
 ## C API updates
 
 ## Implementation improvements