Add Next.js Integration to documentation #381

Hi,
first of all thanks for open source such a great project.
Since I use it with Next.js myself and wanted to have a easier more confortable way to use it with Next.js, I've created a NPM Package for it called next-rybbit ( https://www.npmjs.com/package/next-rybbit ) and open soured it as well to help get more people to start using your awesome analytics software.
It would be nice if you could maybe add it to the documentation since I think it would help people get started with rybbit.

Best regards
Paul