Document how to add peer/group variants as well

[ModProg]

I need to have a variant enabled when the peer is not focused, so I added:

addVariant('peer-unfocused', ':merge(.peer):not(:focus) ~ &')

but to find out how to do this, I had to take a look at the tailwind source code. Maybe that is just my inexperience with css selectors, but I think this would be a nice addition to the documentation here: https://tailwindcss.com/docs/plugins#adding-variants

[RobinMalfait]

Hey! Thank you for your suggestion!
Much appreciated! 🙏

We will definitely add more docs on the new addVariant API. Took note of it and we will make sure that it is present in the future 👍

[hazae41]

While the full documentation is being written, you may add a link to your PR about the new addVariant API; which is quite complete but somewhat hard to find for newcomers 👍