A few friendly Symfony4 updates :)

[weaverryan]

I see you already have #25 about adding a recipe - cool!

I'm including this bundle in an upcoming KnpU course, and I noticed a few things that would be helpful for Symfony 4 users:

1. Add an autowire alias for the Slack Client
2. Update the documentation for Symfony 4. This includes not mentioning AppKernel (or at least, if you DO want to mention it for Symfony 3, adding a note about this), adding a filename above the config block (I think it's ok to say # Symfony 3: app/config/config.yml and on the next line #Symfony 4: config/packages/nexy_slack.yaml, and updating the controller code to show action-autowiring, instead of using the public service (again, you can keep a reference to using the public service for Symfony 3 users, but it should show the Symfony 4 way for sure).

Thanks for the cool bundle! It made for a great and fun example on our tutorial :).

[soullivaneuh]

Hi @weaverryan,

Thanks for the suggestion!

I keep this issue open, any PR will be welcomed! :wink:

[soullivaneuh]

Autowiring is provided by #31