doc: add instructions for upgrading class names to v4 in README

Author: EricGusmao

README.md

@@ -13,6 +13,7 @@
   * [You don't _have_ to upgrade](#you-dont-_have_-to-upgrade)
   * [Upgrade steps](#upgrade-steps)
   * [Troubleshooting](#troubleshooting)
+  * [Upgrading class names for v4](#upgrading-class-names-for-v4)
 - [Developing with Tailwindcss](#developing-with-tailwindcss)
   * [Configuration and commands](#configuration-and-commands)
   * [Building for production](#building-for-production)
@@ -91,21 +92,13 @@ gem "tailwindcss-ruby", "~> 3.4"
 
 First, update to `tailwindcss-rails` v4.0.0 or higher. This will also ensure you're transitively depending on `tailwindcss-ruby` v4.
 
+
 ```html
 # Gemfile
 gem "tailwindcss-rails", "~> 4.0" # which transitively pins tailwindcss-ruby to v4
 ```
-To replace the class names accordingly for v4, go to ``config/tailwind.config.js`` update the ``content`` part to:
 
-```js
-  content: [
-    '../public/*.html',
-    '../app/helpers/**/*.rb',
-    '../app/javascript/**/*.js',
-    '../app/views/**/*.{erb,haml,html,slim}'
-  ],
-```
-(Just add an additional ``.`` to all the paths referenced)
+If you want to migrate class names for v4 (optional), apply this [step](#upgrading-class-names-for-v4) and continue this guide.
 
 Add the following line to the `.gitignore` file:
 
@@ -207,6 +200,20 @@ We know there are some cases we haven't addressed with the upgrade task:
 
 We'll try to improve the upgrade process over time, but for now you may need to do some manual work to upgrade.
 
+### Upgrading class names for v4
+
+Before running the upgrade task, go to ``config/tailwind.config.js`` update the ``content`` part to:
+
+```js
+  content: [
+    '../public/*.html',
+    '../app/helpers/**/*.rb',
+    '../app/javascript/**/*.js',
+    '../app/views/**/*.{erb,haml,html,slim}'
+  ],
+```
+(Just add an additional ``.`` to all the paths referenced)
+
 
 ## Developing with Tailwindcss