Add mention about manual opening of browser tab, not automatical

[Andrii256]

Hello
I found the same issue as here #72

I used to just wait when tab in browser will opens automatically. So it misleads a bit.
This is not critical, but it would be good if you add mention about it in documentation :)

[AshlandWest]

I second (third?) this request, but I suggest it be resolved by opening a browser tab at the end of the start script rather than noting the likely-expected functionality doesn't exist.
This would also match the default functionality of create-react-app and other popular packages.