Add use-npm CLI flag docs (#17803)
Add use-npm CLI flag docs (#17803)

Related issue: https://github.com/vercel/next.js/issues/10647#issuecomment-623703149

Sometimes we have both Yarn and NPM installed and want to explicitly bootstrap an app with the CLI using NPM. Having this flag documented could help people understand the package manager behavior and select NPM if that's their preference.
flybayer/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js