docs: Add CI usage instructions to README

[chriscrabtree]

What I thought was an issue related to configuration wasn't. :-)

It turned out to be me not running the correct steps for what tailwind_formatter needs in CI. The existing documentation was helpful in figuring this out, but I did not find it quite explicit enough to make it easy. Indeed, it wasn't until reading through the code in the Order module did I realize what needed to happen.

This PR aims to smooth adoption of your valuable work by adding an explicit section to the README for usage in CI.