docs: add STRUCTURE.md explanation & guidelines

CONTRIBUTING.md

@@ -13,6 +13,7 @@ This document is for developers or programmers contributing to the source code o
     - [Prerequisites](#prerequisites)
     - [Build](#build)
     - [Run](#run)
+  - [Project Structure](#project-structure)
   - [Fastlane](#fastlane)
   - [Workflow](#workflow)
     - [Coding Guidelines](#coding-guidelines)
@@ -92,6 +93,10 @@ test                  # Typechecks + Unit + integration tests
 
 **NOTE**: If having issues starting the emulator, try cleaning your build files and running `npm i`.
 
+## Project Structure
+
+For information on how this project is structured, please see the [./docs/STRUCTURE.md](structure) documentation.
+
 ## Fastlane
 
 `fastlane` is automation tool for deployments for iOS and Android. We use this to sign code and bump versions for releases. To setup `fastlane` locally you need to have [`ruby`](https://www.ruby-lang.org/en/documentation/installation/)installed.

STRUCTURE.md

@@ -0,0 +1,100 @@
+# Project Structure
+
+Welcome to our app's codebase! Shabad OS Mobile is a [React Native](https://reactnative.dev/) app for both iOS and Android.
+
+This is a living document to capture and share the stucture of the app to make it easier to understand where existing code lives and where new code should go.
+
+## Project Root
+
+The following is a reduced, annotated listing of the project's root directory (generated with `tree -L 1 -F --dirsfirst -a`) to provide a high-level overview of purpose.
+
+```
+.
+├── android/        Native Android project
+├── assets/         All custom fonts & global images
+├── config/         Environmnent specific configuration variables
+├── docs/           Supporting documentation & corresponding assets
+├── fastlane/       App-specific build & release code driven by Fastlane
+├── ios/            Native iOS project
+├── metadata/       Cross-platform app store metadata (e.g. app icon)
+├── release-notes/  Version release notes
+├── src/            Source code for app
+├── test/           Shared test utilities
+└── index.js        App entrypoint for React Native code
+```
+
+As a contributor, you'll likely spend the majority of your time inside the [src](#source-code) folder.
+
+## Configuration
+
+Dotenv configuration files are stored inside the `config` folder, with a file per environment. More information can be found inside the [Configuration README document](./config/README.md).
+
+## Source Code
+
+The `src` directory contains all source code and is split into the following high-level folders:
+
+```
+src
+├── components/
+├── helpers/
+├── screens/
+├── services/
+├── themes/
+├── types/
+├── App.spec.tsx
+└── App.tsx
+```
+
+### Components
+
+Components are the fundamental visual building blocks of the app and can be composed to form more complex components.
+
+Styles and internationalisation strings should be colocated or inlined with the corresponding components.
+
+Tests should be colocated as `[Component].spec.tsx` file.
+
+### Screens
+
+Screens are the views ultimately presented to a user, comprised of templates and other components. If components used by the screen are not shareable across the app, they should be colocated with the screen.
+
+Integration tests for screen should be colocated and written as `index.int.spec.tsx` in the screen's folder.
+
+### Helpers
+
+Helpers are generic, reusable, cross-cutting utilities. These should be small, lean, and composable.
+
+### Services
+
+Services are cross-cutting providers of specific functionality to the app. This can contain business logic, data accessors, and other providers that the app will use. Services can compose other services.
+
+### Styles
+
+Styles should typically be inlined with the concerned component, however in cases where shared or cross-cutting styling is required, the `themes` folder exists.
+
+### Types
+
+Types should typically be inlined or colocated with their concerns, however some types may be required globally (e.g. data definitions or utility types). These types should sit in the `types` folder.
+
+## Tests
+
+Tests should always be colocated with their concerns, and follow the [Arrange-Act-Assert](https://jefflau.dev/arrange-act-assert-how-to-test-react-components/) model. Test utilities may sit in the `test` folder.
+
+#### Document TBC
+
+### Unit Tests
+
+### Integration Tests
+
+### E2E Tests
+
+End-to-end tests are tbd!
+
+## Native Code
+
+## Assets
+
+## Building & Fastlane
+
+## Pipelines
+
+## Release Notes