Update README for Rails 6 with webpacker

[hostingnuggets]

Now that Rails 6 is out since a few months it would be great if the Installation paragraph in this project's README file could be updated to explain how to use this gem with Rails 6 and webpacker. Unfortunately the current instructions are not valid anymore for a new Rails 6 project using webpacker.

[airblade]

Please could you send a pull request? Alternatively, would you like to take over the project (#53)?