Prep docs and post-install message for final v4.0.0 release

[flavorjones]

1. Clean up the README's upgrade docs.
   @EricGusmao please take a look when you get a moment. 🙏
2. Add a post-install message pointing people to the upgrade guide:

   == Upgrading to Tailwind CSS v4 ==
   
   If you are upgrading to tailwindcss-rails 4.x, please read the upgrade guide at:
   
     https://github.com/rails/tailwindcss-rails/blob/main/README.md#upgrading-your-application-from-tailwind-v3-to-v4
   
   If you're not ready to upgrade yet, please pin to version 3 in your Gemfile:
   
     gem "tailwindcss-rails", "~> 3.3.1"
   
   

[EricGusmao]

I think from this line up to line 198 should be part of the main Upgrade steps, since the upgrade rake task runs the upgrade tool, the tool will fail if the user has not installed the packages referenced in the config file.

[EricGusmao]

These steps are necessary not only to migrate the class names but also to convert the configuration to the new CSS format, such as plugins and user custom configurations that might reference to tailwind default theme.

[flavorjones]

The upstream upgrade tool only fails if additional packages have been added and can't be found. A vanilla rails app doesn't need these steps, and they are complex and frankly hard to visually parse and follow as currently written.

I think if this is truly "required" then we should automate it. And if it's not worth automating we should keep it marked as optional. And if it's hard to automate then we need to file issues upstream to have the Tailwind project make the tool work better.

WDYT?

[flavorjones]

Like, I'm really having a hard time understanding why these steps are necessary, and I'm sorry about that. The rake task works fine for me on my personal projects. And I understand that more complex setups need more complex steps, but then I think it's OK to leave those steps as a separate section. If you want to suggest alternative wording for the section maybe that would split the difference?

[EricGusmao]

Makes senses. Another suggestion is to move up the following excerpt and place it below Upgrade Steps title, maybe something like this:

Warning

In setups without JavaScript tooling, the update process may fail to fully migrate tailwind.config.js because the tool assumes that the imported packages (e.g., tailwind plugins) are installed via a package manager, allowing them to be called. In this case, you should try following the instructions in Updating CSS class names for v4 which will install the needed javascript packages for the updater.

So the reader is aware upfront and doesn't have to go all the way down the guide to find a solution if something goes wrong.

[flavorjones]

I like this, will update the PR. Thank you!! 🙏❤️

[EricGusmao]

Great!!

Side note: I sent you an email. You might want to check it when you have a moment.

[flavorjones]

I've updated the PR with some more README changes. LMK what you think if you get a moment, otherwise I'll merge over the weekend.

[EricGusmao]

It looks perfect to me.