Update decorators.md

This PR adds additional info for decorators section to save others time figuring out why default `webpacker:typescript` installation (which uses `stage2` AFAIR behavior) does not work with `@controller` decorator.

Author: lcmen

docs/_guide/decorators.md

@@ -3,7 +3,7 @@ chapter: 4
 subtitle: Using TypeScript for ergonomics
 ---
 
-Decorators are used heavily in Catalyst, because they provide really clean ergonomics and makes using the library a lot easier. Decorators are a special, (currently) non standard, feature of TypeScript. You'll need to turn the `experimentalDecorators` option on inside of your TypeScript project to use them.
+Decorators are used heavily in Catalyst, because they provide really clean ergonomics and makes using the library a lot easier. Decorators are a special, (currently) non standard, feature of TypeScript. You'll need to turn the `experimentalDecorators` option on inside of your TypeScript project to use them (if you're using `@babel/plugin-proposal-decorators` plugin, you need to use [`legacy` option](https://babeljs.io/docs/en/babel-plugin-proposal-decorators#legacy)).
 
 You can read more about [decorators in the TypeScript handbook](https://www.typescriptlang.org/docs/handbook/decorators.html), but here's quick guide: