Add `nvm` install instructions in Readme for manual setup section

[KrishPatel13]

What kind of change does this PR introduce?

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• [x] My code follows the style guidelines of OpenAdapt
• [x] I have performed a self-review of my code
• [x] If applicable, I have added tests to prove my fix is functional/effective
• [x] I have linted my code locally prior to submission
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (e.g. README.md, requirements.txt)
• [x] New and existing unit tests pass locally with my changes

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