feat: add contribution doc

CONTRIBUTING.md

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+# Contribute to BehatMessengerContext
+
+Thank you for contributing!
+
+Before we can merge your Pull-Request here are some guidelines that you need to follow.
+These guidelines exist not to annoy you, but to keep the code base clean,
+unified and future proof.
+
+## Dependencies
+
+We're using [`composer/composer`](https://github.com/composer/composer) to manage dependencies
+
+## Coding Standard
+
+This project uses [PHP CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) to enforce coding standards.
+The coding standard rules are defined in the **phpcs.xml.dist** file (part of this repository).
+
+Your Pull-Request must be compliant with the said standard.
+To check your code you can run `composer run code-style`. This command will give you a list of violations in your code (if any).
+
+The most common errors can be automatically fixed just by running `composer run code-style-fix`.
+
+[coding standard homepage]: https://github.com/doctrine/coding-standard
+
+## Static analysing tools
+
+This project uses [PHPStan](https://github.com/phpstan/phpstan) to find errors in code without running it.
+The analyser configuration is defined in the **phpstan.neon.dist** file (part of this repository).
+
+Your Pull-Request must be compliant with PHPStan rules.
+To check your code you can run `composer run phpstan`. This command will give you a list of violations in your code (if any).
+
+If error can't be fixed you should add it to `ignoreErrors` in  **phpstan.neon.dist**
+
+## Unit-Tests
+
+Please try to add a test for your pull-request. This project uses PHPUnit as testing framework.
+
+You can run the unit-tests by calling `composer run phpunit`.
+
+New features without tests can't be merged.
+
+## Issues and Bugs
+
+To create a new issue, you can use the GitHub issue tracking system.
+
+## Getting merged
+
+Please allow us time to review your pull requests. We will give our best to review
+everything as fast as possible, but cannot always live up to our own expectations.
+
+Pull requests without tests most probably will not be merged.
+Documentation PRs obviously do not require tests.
+
+Thank you very much again for your contribution!