Add more docs, refactor formatters

Author: IonBazan

README.md

@@ -84,6 +84,8 @@ composer diff -p # include platform dependencies
 composer diff -f json # Output as JSON instead of table
 ```
 
+You can find more documentation in the [docs](docs) directory.
+
 ### Strict mode
 
 To help you control your dependencies, you may pass `--strict` option when running in CI. If there are any changes detected, a non-zero exit code will be returned.
@@ -99,6 +101,11 @@ Exit code of the command is built using following bit flags:
 
 You may check for individual flags or simply check if the status is greater or equal 8 if you don't want to downgrade any package.
 
+# Contributing
+
+Composer Diff is an open source project that welcomes pull requests and issues from anyone. 
+Before opening pull requests, please consider reading our short [Contribution Guidelines](docs/CONTRIBUTING.md).
+
 # Similar packages
 
 While there are several existing packages offering similar functionality:

docs/CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing guidelines
+
+Any contributions (issues, pull requests, code review, ideas, etc.) are welcome. 
+Here are a few guidelines to be aware of:
+
+- Include tests for any new features or bug fixes.
+- All new features should target the `main` branch.
+- All code must follow the project coding style standards which will be enforced by [StyleCI](https://styleci.io/).
+
+Since this project has rather strict coding standards, which include:
+  - PHPStan level 6
+  - 100% test coverage
+  - 100% MSI (Mutation Score Indicator)
+  - Compatibility with PHP 5.3.2 up to 8.0 and newer
+
+It might be a bit challenging to get started but fret not, 
+as the maintainers will be happy to assist you should you have any questions or need help with your contribution.
+Even if the tests fail, don't worry, as the maintainers will help you fix them.
+
+## Getting started
+
+1. Fork the repository on GitHub.
+2. Clone your fork locally.
+3. Run `composer install` to install the dependencies.
+4. Create a new branch for your feature or bug fix.
+5. Write code and tests for your new feature or bug fix.
+6. Run the tests (`vendor/bin/simple-phpunit`) to be sure everything is working.
+7. Push your branch to your fork on GitHub.
+8. Create a pull request to the `main` branch.
+9. Wait for the maintainers to review your pull request.
+
+## Testing against a real project
+
+Sometimes it's easier to check your changes when you have an actual project where a bug occurs rather than reproducing it in a test.
+While this package works as a Composer plugin, it might seem difficult to test your changes against a real project.
+
+Consider a following example:
+
+- `~/work/my-project` - path to my project
+- `~/work/composer-diff` - path to this repository
+
+Running the `composer-diff` command with your changes against your project is as simple as:
+
+```shell
+cd ~/work/my-project # Navigate to your project directory
+~/work/composer-diff/composer-diff # Run the composer-diff command from this repository
+```
+
+You can specify any other options as well like `--no-dev`, `--with-platform`, etc.