docs: Better inform users about supported Nextjs versions

[ahuseyn]

Documentation should reflect the supported Next.js and React versions and other system requirements that people might need to know, before using the package.

Acceptance Criteria

• README.md includes supported Nextjs versions.
• https://github.com/wpengine/faustjs.org includes supported Nextjs versions

[josephfusco]

Related in the sense of public versioning that needs updating

wpengine/faustjs.org#371