chore(repo): Add CONTRIBUTORS.md (#115)

Adds initial contribution guidelines for the repo.

CONTRIBUTING.md

@@ -0,0 +1,39 @@
+# Welcome to the Celest repo
+
+> **Contributors should read these guidelines in full before submitting any changes for review.**
+
+This repository contains the source code for many components of Celest, a project that aims to provide a simple and easy-to-use platform for developers to create and deploy Dart and Flutter applications to the cloud.
+
+## Contributing
+
+### Get Started
+
+To get started contributing to Celest, clone the repository using the `git clone` command:
+
+```bash
+$ git clone https://github.com/celest-dev/celest.git
+```
+
+The monorepo uses [melos](https://pub.dev/packages/melos) for managing dependency linking and versioning. Before starting, ensure you have melos installed globally:
+
+```bash
+$ dart pub global activate melos
+```
+
+Then run `melos bootstrap` to link all packages together:
+
+```bash
+$ melos bootstrap
+```
+
+### Submitting a Pull Request
+
+Ensure that any changes made to a package have accompanying tests. To run tests for a package, navigate to the package directory and run the following command:
+
+```bash
+$ dart test
+```
+
+Pull request titles should follow [convential commit](https://www.conventionalcommits.org/en/v1.0.0/) format `type(scope): Title` where `type` is the type of change being made (e.g. `feat`, `chore`, `refactor`, etc) and `scope` is the scope of the change (e.g. `auth`, `core`, or `repo` for repo-wide changes). 
+
+For example, a pull request that adds a new feature to Celest Auth should have a title like `feat(auth): Passkey management`.