Add contributing guidelines to CONTRIBUTING.md

Added guidelines for contributing to the project, including setup instructions for Firebase Emulator Suite and mobile app development.

Author: Salint

CONTRIBUTING.md

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+# How to contribute
+
+TwoAxis Finance is built entirely on Firebase, and the mobile client is built using Flutter.
+
+## Running the Firebase Emulator Suite
+In order to contribute, you must use the Firebase Emulator Suite.
+
+- Make sure that you have Firebase Tools CLI installed: `npm install -g firebase-tools`
+- Run Firebase emulators locally: `firebase emulators:start`
+
+## Running the mobile app
+- Open the `mobile/` directory in Android Studio (or IDE that supports Flutter).
+- Run `main.dart` in development mode (`assembleDebug`)
+
+Note: The mobile client connects to the Firebase Emulators in debug mode only (`kDebugMode`). You may change this in `lib/main.dart`, but do not commit your changes.
+
+## Pull Requests
+Your pull requests will be reviewed throughly, please submit many small pull requests rather than one big one. 
+Make sure to assign proper labels to your pull request as it will impact the review.
+Include a description with your pull request that includes an overview of what the pull request changes.