chore: Add instructions for cross subdomain development

[robbie-c]

Changes

I'm trying to debug some logic around cross-subdomain tracking, and figure out how to do this while running on localhost. I thought I would add some docs for other people (or my future self) who want to try this

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Aug 1, 2024 4:09pm

[github-actions]

Size Change: 0 B

Total Size: 1.16 MB

ℹ️ View Unchanged

Filename	Size
dist/array.full.js	333 kB
dist/array.js	154 kB
dist/exception-autocapture.js	10.4 kB
dist/main.js	154 kB
dist/module.js	154 kB
dist/recorder-v2.js	110 kB
dist/recorder.js	110 kB
dist/surveys-preview.js	59.5 kB
dist/surveys.js	65.6 kB
dist/tracing-headers.js	8.26 kB
dist/web-vitals.js	5.79 kB

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