updates to readme (recommendation)

[dudmuck]

• minimum version of nodejs required, since some linux distrubutions have older version that wont work with ncs-app-index ... nodejs-v12 wasnt working for example, but a more recent version is ok

• how to open vscode's settings.json, which appears to be --> ctrl-shift-P and search for "Preferences: Open User Settings (JSON)"

• in section "Verify index in the extension" step 2: correct URL for nrf-connect.appIndexUri in settings.json should be http://localhost:3000/ncs-app-index Because this is whats used above in the "verify website locally section"