Add Documentation On Disabling Cookies

[bharget]

Thank you for this gem! This adds documentation on disabling the before action for setting cookies for particular Rails controller/action. I have a Rails app that serves some API routes and some full MVC routes, in which case I'd rather not set Ahoy.api_only. Hoping this helps someone else looking for the same thing!

Let me know what you think about the placement in the README. Most likely this would always be paired with

skip_before_action :track_ahoy_visit

but I didn't want to emphasize this method since it's probably not as common of a use case.