README: Mention and link to devtools-next

[SimonEast]

What problem does this feature solve?

Hi team,

I just spent some time digging around to try and understand the floating Vue DevTools panel that was showing in my dev environment and I initially thought it was related to this repo, but later realised it was part of vuejs/devtools-next. I'm surprised there's no mention of that in the current devtools README.

Would it be OK if there was a mention and link to it, perhaps with a sentence or two about the differences and which is best used in common scenarios?

If my understanding is correct, devtools-next actually provides more features, but only works on Vue 3, and does not yet have a working Chrome Extension - instead it needs to be activated per project by installing the necessary packages. Is that right?

I'm happy to submit a PR if others also think this would be helpful, and if someone can provide some clarity on the differences between the two.