Add note about supported Rails versions

README.md

@@ -150,6 +150,14 @@ Premailer::Rails::Hook.perform(mail)
 
 This will modify the email in place, useful e.g. in tests.
 
+## Supported Rails Versions
+
+This gem is tested on Rails versions 5 through 7.
+
+For Rails 7, it only support the classical Sprockets asset pipeline. It does not yet support [Propshaft](https://github.com/rails/propshaft).
+
+If you're looking to integrate with Webpacker, check out [these instructions](https://github.com/fphilipe/premailer-rails/issues/232#issuecomment-839819705).
+
 ## Small Print
 
 ### Author