README clarify javascript:build may also need to be run if test:prepare is not defined

[ajaynomics]

When a user invokes rails test or uses a test framework that does not define a test:prepare Rake task, css:build must be run manually. This is already documented in the README.

If jsbundling-rails is also being used, javascript:build may also need to be run (ie, esbuild + tailwind).

If either css:build or javascript:build is required and is not run, minitest reports the same error:

ActionView::Template::Error: The asset "application.css" is not present in the asset pipeline.

This PR aims to improve the documentation to avoid others chasing their tail for a few hours like I did.