Add yarn dedupe recommendation

[yjaaidi]

We should recommend using yarn dedupe for yarn users.

@babel/* & @angular-devkit/build-angular both depend on browserslist with different version ranges but yet compatible.
Without running yarn dedupe, @babel/* & @angular-devkit/build-angular might end up using different browserslist version thus causing confusing errors (e.g. Unknown version 110 of chrome (While processing: "base$0$0")).
We have to run yarn dedupe to fix this.

Cf. angular/angular-cli#25165
Cf. angular/angular-cli#24837