Lacking documentation of using postcss with `tailwindcss-rails

[leewaa]

Unfortunately it is not totally clear what is required in order to use PostCSS with this Gem.

The Readme simply states to add the config file but does not describe any other prerequisites or setup requirements.

There are very many ways in which one can add postcss with many Rails applications running different setups (webpacker/sprockets/importman/yarn/node/etc) but this is not clear enough in the readme. It would be nice if this section had more information and explicit setup instructions to use postcss with tailwindcss-rails.

Simply installing this gem, running ./bin/rails tailwindcss:install and then adding the postcss.config.js example file as described, does not work out of the box. Would be very helpful with extended information.