Update CONTRIBUTING guide (#2261)

* Revise contributing guide to incorporate recent developments

* Implement PR review feedback

Co-authored-by: Djordy Koert <djordy.koert@gmail.com>

---------

Co-authored-by: Djordy Koert <djordy.koert@gmail.com>

CONTRIBUTING.md

@@ -1,28 +1,50 @@
-Contributing
-============
+# Contributing
 
-First of all, **thank you** for contributing, **you are awesome**!
+First of all, **thank you** for contributing, **you are awesome**! With your contribution, you not only improve this
+bundle, but also become part of a [great community](https://github.com/nelmio/NelmioApiDocBundle/graphs/contributors)
+maintaining it.
 
-Here are a few rules to follow in order to ease code reviews, and discussions before
-maintainers accept and merge your work.
+## Guidelines
 
-You MUST follow the [PSR-1](http://www.php-fig.org/psr/psr-1/) and
-[PSR-2](http://www.php-fig.org/psr/psr-2/). If you don't know about any of them, you
-should really read the recommendations. Can't wait? Use the [PHP-CS-Fixer
-tool](http://cs.sensiolabs.org/).
+Here are a few guidelines to follow in order to ease code reviews, and discussions before maintainers can accept and
+merge your work. Thank you!
 
-You MUST run the test suite (run `composer update`, and then execute `composer run phpunit`).
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
+"RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be interpreted as described in
+[RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119).
 
-You MUST write (or update) unit tests.
+### Code Style
 
-You SHOULD write documentation.
+Code MUST adhere to all rules outlined in the
+[Symfony Coding Standards](https://symfony.com/doc/current/contributing/code/standards.html) as
+defined by the [@Symfony](https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/blob/master/doc/ruleSets/Symfony.rst) rule set,
+utilized by the [PHP-CS-Fixer](https://cs.symfony.com) tool.
 
-Please, write [commit messages that make
-sense](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html),
-and [rebase your branch](http://git-scm.com/book/en/Git-Branching-Rebasing)
-before submitting your Pull Request.
+You SHOULD run `composer run phpcs-check` to check for any violations. You SHOULD run `composer run phpcs-fix` to
+fix any potential issues.
 
-Also, while creating your Pull Request on GitHub, you MUST write a description
-which gives the context and/or explains why you are creating it.
+### Code Quality
 
-Thank you!
+You MUST use the static analysis tool [PHPStan](https://phpstan.org/) to analyse any newly added or revised code within
+this bundle.
+
+You MUST run `composer run phpstan` to check for any violations. You MUST fix all violations related to any newly added
+or revised code.
+
+### Tests
+
+You MUST write (or update) unit and/or functional tests for any newly added or revised functionality within this bundle.
+
+You MUST validate newly added or revised tests by running `composer run phpunit`.
+
+### Documentation
+
+You SHOULD write (or update) documentation.
+
+You SHOULD write
+[commit messages that make sense](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
+
+You MUST [rebase your branch](https://git-scm.com/book/en/v2/Git-Branching-Rebasing) before submitting your Pull Request.
+
+While creating your Pull Request on GitHub, you MUST write a description which gives the context and/or explains why you
+are creating it.