docs: update readme for nextjs and webpack plugin

[aayushchugh]

What changed / motivation ?

Added documentation for installation process in README file of @stylexjs/nextjs-plugin and @stylexjs/webpack-plugin

Linked PR/Issues

Fixes #164

Additional Context

Screenshots, Tests, Anything Else

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

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[nmn]

Please mention that the @/ alias is unsupported at the moment.

[aayushchugh]

@nmn Added that, please check

[nmn]

Thank You!