Aspect ratio plugin: Add note about safelist for aspect classes?

[jamesmacwhite]

It might be me, being new to tailwind, but I was smacking my head against the wall for a while thinking the plugin wasn't working, when in fact it was, but the aspect classes where being generated from a dynamic source i.e. database.

I needed to the aspect- class prefix to the safelist configuration to avoid it being stripped and not showing up most of the time.

A note in the README, given this might be a use case more common, i.e. a field containing data being wrapped with the aspect classes.

In relation to: https://github.com/tailwindlabs/tailwindcss-aspect-ratio

Fully acknowledging that this is well documented in the main docs: https://tailwindcss.com/docs/content-configuration#safelisting-classes but I didn't make the connection for a while, until I started testing outside of dynamic template driven by fields/database.