Add missing step to README for installing Horizon

[SketchNI]

Adding HorizonServiceProvider to config/app.php was missing in the installation steps.

[bradleybensmith]

Laravel 5.5 does this automatically - not needed.

[SketchNI]

Does L5 do this automatically for any composer package?

[bradleybensmith]

No, the package's composer.json has to be setup for it. In this case:

"extra": {
        ...
        "laravel": {
            "providers": [
                "Laravel\\Horizon\\HorizonServiceProvider"
            ]
        }
    },

[SketchNI]

That's pretty neat. Can't wait for that. I'll close this PR.

[bradleybensmith]

See https://laravel-news.com/package-auto-discovery for more info :)