Next.js should warn and encourage upgrade on React <16.10

[Timer]

Next.js is fully compatible with React versions prior to 16.10, however, Fast Refresh does not work, and your development server will do a full reload on every edit.

This is a suboptimal experience, so we should warn the user when we detect an old React version and encourage them to upgrade, explaining the benefits, etc!

[todortotev]

@Timer I have open PR for this issue.