Update contribution guide

[chrisglein]

Current contribution guide is here: https://github.com/microsoft/react-native-gallery/wiki/Contributing-to-React-Native-Gallery
Mostly it covers pretty broad contribution topics, not the quirks specific to this repo. That's a fine doc to have... but we really need to capture what you need to know if you're going to work on this app.

Things like:

• Running yarn test:update
• Clearer callout of timeZoneOffsetInSeconds problem (it's a bit buried as is): #441
• Linting expectations (yarn lint clean)
• Accessibility checking with Accessibility Insights 🧪
• Anything else you might hit in the pipelines
• Hotspots for manual testing
• Platform-specific code
• How to add a new sample page (note: covered in the readme)
• App architecture overall