Update Readme to show Rails Credentials as example

[MikeRogers0]

Am I OK to update the README.md Usage section to use Rails credentials instead of Rails secrets?

It would replace the code sample, with:

config.omniauth :spotify, Rails.application.credentials.spotify[:client_id], Rails.application.credentials.spotify[:client_secret], scope: %w(
  playlist-read-private
  user-read-private
  user-read-email
).join(' ')

[masterkain]

👍