Clarify configuration requirements, provide examples

[yuriybabenko]

It is not clear from the Readme and other available documentation what configuration is required and where.

I'm working on handling GitHub hooks, and it has been frustrating to run into error after error while attempting to integrate the "drop-in" package.

For example, nowhere does it specify that my application's /config/services.php needs to be updated to include a webhook_secret configuration value, and that the same value needs to be configured in the GitHub webhook:

'github' => [
  'webhook_secret' => env('GITHUB_WEBHOOK_SECRET'),
],

I'd imagine there are similar requirements for other providers, too.

Improving the docs in this regard would go a long way towards easier package adoption.