Mention how to use webpacker in updated Rails apps [ci skip]

guides/source/upgrading_ruby_on_rails.md

@@ -78,6 +78,20 @@ Upgrading from Rails 5.2 to Rails 6.0
 
 For more information on changes made to Rails 6.0 please see the [release notes](6_0_release_notes.html).
 
+### Using Webpacker
+
+[Webpacker](https://github.com/rails/webpacker)
+is the default JavaScript compiler for Rails 6. But if you are upgrading the app, it is not activated by default.
+If you want to use Webpacker, then include it in your Gemfile and install it:
+
+```ruby
+gem "webpacker"
+```
+
+```sh
+bin/rails webpacker:install
+```
+
 ### Force SSL
 
 The `force_ssl` method on controllers has been deprecated and will be removed in