docs(readme): use pnpm to manage node versions instead of nvm

[MFarabi619]

Problem Statement

Current README encourages the use of nvm.

This is unneccessary, as the project uses pnpm, which is already capable of managing node versions via pnpm env use --global ... since version 6.

Proposed Solution

Explore pnpm to manage node versions instead of nvm, and update readme accordingly.

https://pnpm.io/cli/env