chore(docs): add developer guide for contributors

Author: imdhemy

DEVELOPER_GUIDE.md

@@ -0,0 +1,88 @@
+# Developer guide
+
+We're glad you're interested in contributing to our project! This guide will help you get started with the development
+process.
+
+## Getting Started
+
+[Fork the repository](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo)
+and [clone it](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository) to your
+local machine.
+
+### Prerequisites
+
+The preferred way is to use [Docker](https://www.docker.com/get-started) and Make.
+
+Alternatively, ensure you have the following installed:
+
+- **PHP** You can know which version is required by checking the `composer.json` file.
+- **Composer** Dependency manager for PHP. [Get Composer](https://getcomposer.org/download/)
+
+### Starting the Development Environment
+
+If you're using Docker & Make, simply run:
+
+```bash
+make start
+```
+
+This command will build the docker image, start the container, and take you inside the container bash shell so that you
+can directly run commands.
+
+### Installing Dependencies
+
+Inside the docker container or your local environment, run:
+
+```bash
+composer install
+```
+
+### Running Code Quality Tools
+
+To ensure the code quality and CI compliance, we are using the following tools:
+
+#### PHPUnit for testing
+
+To run the full test suite, execute:
+
+```bash
+composer test
+```
+
+#### PHP CS-Fixer for code style
+
+To check and fix code style issues, run:
+
+```bash
+composer cs-fix
+```
+
+#### Psalm for static analysis
+
+To perform static analysis, run:
+
+```bash
+composer psalm
+```
+
+## Code Contribution Guidelines
+
+When I (Dhemy) started this project, like any developer, I had my own coding style and preferences. Over time, I
+acquired more knowledge and experience; additionally PHP itself evolved. As a result, the codebase might reflect a mix
+of styles and practices.
+
+Currently, We are following the principles
+of [Clean Architecture](https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html).
+See [this issue](https://github.com/imdhemy/google-play-billing/issues/236) for the current state of the codebase and
+the planned improvements.
+
+## Pick your work
+
+The best way to start contributing is to check the issues list. Pick an issue that interests you and start working on
+it. If you didn't find any issue, you can always propose new features or improvements by opening a new issue.
+
+## Weekly meeting
+
+We have a weekly meeting every Friday at 16:00 Berlin time. Feel free to join us
+on [Google Meet](https://meet.google.com/zgo-wtpp-jfh) to discuss the ongoing
+work and any questions you might have.