Add post install message pointing to the changelog and new upgrade guide

There's some additional information in the wiki upgrade guide for those
interested, but most of it is covered in the changelog and should
suffice.

The post install message should help guide people upgrading to make sure
they know what to do in this new version, since some may be using Turbo
out there with custom responders and failure apps and those would have
to be removed in order to use these new changes fully. Hopefully that's
enough of a nudge for them.

CHANGELOG.md

@@ -20,6 +20,7 @@
     * If you have created a custom responder and/or failure app just to customize responses for better Hotwire/Turbo integration, they should no longer be necessary.
     * `:turbo_stream` is now treated as a navigational format, so it works like HTML navigation when using Turbo. Note: if you relied on `:turbo_stream` to be treated as a non-navigational format before, you can reconfigure your `navigational_formats` in the Devise initializer file to exclude it.
     * OmniAuth "Sign in with" links were changed to buttons that generate HTML forms with method=POST, instead of using link + method=POST that required rails/ujs to work. Since rails/ujs is no longer the default for new Rails apps, this allows the OmniAuth buttons to work in any scenario, with or without rails/ujs and/or Turbo. This only affects apps that are using the default `devise/shared/_links.html.erb` partial from Devise with OmniAuth enabled.
+    * Check [this upgrade guide](https://github.com/heartcombo/devise/wiki/How-To:-Upgrade-to-Devise-4.9.0-[Hotwire-Turbo-integration]) for more detailed information.
 
 ### 4.8.1 - 2021-12-16