Added a note about prefixes

[smebberson]

After failing to get containers to work, I discovered #13 and that I was suffering from the same problem.

This PR adds a little note to the documentation to make it clear, because it wasn't obvious to me either.

[adamwathan]

Hey thanks! This isn't something we normally document in plugins because it's always been the case that if you enable a prefix, every Tailwind class needs to be prefixed to work. I'm going to merge it though because with the special @ character I think it's maybe less obvious where/how to use the prefix with the @container class. Thanks.

Just a heads up — the prefix was also missing on the @lg/main:underline class in the PR, so I've fixed that otherwise the documentation would have been incorrect 👍