Update local testing doc to show our own workflow

[timneutkens]

As per https://twitter.com/pierrevanmart/status/1109030783405248513

[github-actions]

Stats from current PR

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	zeit/next.js canary	timneutkens/next.js add/instructions-for-local-test	Change
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Client _app Size	3.03 kB	3.03 kB	✓
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