[docs] guidance on optimizing css for production

[davidsainez]

Hello, I'm looking for guidance on optimizing payload sizes for production.

I see a short section in the README, but a more detailed starting point would be useful.

My main concern is any caveats to watch for when minimizing and tree-shaking. Ideally, I would like my includes to be fairly general (for ease of development) and then prune any unused styles when building for production. I assume this guidance would also help other consumers of this library. Thanks all for the wonderful code.