Add note about imagemagick / libvips support being required

Author: lewispb

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_2.rb.tt

@@ -13,7 +13,7 @@
 # Adds image/webp to the list of content types Active Storage considers as an image
 # Prevents automatic conversion to a fallback PNG, and assumes clients support WebP, as they support gif, jpeg, and png.
 # This is possible due to broad browser support for WebP, but older browsers and email clients may still not support
-# WebP.
+# WebP. Requires imagemagick/libvips built with WebP support.
 #++
 # Rails.application.config.active_storage.web_image_content_types = %w[image/png image/jpeg image/gif image/webp]