Improvements to README

[karasowles]

I'd like the README to be clearer. I asked Copilot Agent mode to do it, here's the PR based on the text it sent:

#296

@AndreaGriffiths11 or @abbycabs could you review and make sure it's not misleading folks?

[ashleywolf]

Addressed in #380 — README now includes Node.js v20 requirement, clearer Getting Started section, and updated commands from yarn to npm. Thanks for flagging this, @karasowles!