Write development-specific documentation

[achou11]

It'd be wise to include a file that has documentation specifically geared towards working on this project in development. things that are probably worth documenting (amongst other things):

• Environment variables

• Backend build process (e.g. Rollup for bundling, downloading native prebuilds, placing things in the correct places for NodeJS Mobile React Native)

• Commonly run Expo commands, such as expo prebuild --clean

• Useful ADB commands (e.g adb logcat, adb reverse tcp:8081 tcp:8081, etc.)

• How translations generally work

• Polyfill generation script

• generating runnable apks locally (e.g. SENTRY_DISABLE_AUTO_UPLOAD=true eas build --profile test --platform android --local)