Add contribution guidelines

CONTRIBUTING.md

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+# Contributing to Behat API Extension
+
+If you want to contribute to the Behat API Extension please follow the following guidelines.
+
+## Running tests
+
+Behat API Extension has both [Behat](http://docs.behat.org/) and [PHPUnit](https://phpunit.de/) tests, and when adding new features or fixing bugs you are required to add relevant test cases.
+
+The Behat tests requires a web server hosting the `features/bootstrap/index.php` script. A quick and easy alternative is to use PHP's built in web server:
+
+    php -S localhost:8080 -t ./features/bootstrap
+
+After this has been started you can execute the test suites by running:
+
+    composer test
+
+If you want to run the suites separately they can be executed like this:
+
+    ./vendor/bin/behat --strict
+    ./vendor/bin/phpunit
+
+## Reporting bugs
+
+Please use the [issue tracker on GitHub](https://github.com/imbo/behat-api-extension/issues) for this.
+
+## Submitting a pull request
+
+If you want to implement a new feature, fork this project and create a feature branch called `feature/my-awesome-feature`, and send a pull request. The feature needs to be fully documented and tested before it will be merged.
+
+If the pull request is a bug fix, remember to file an issue in the issue tracker first, then create a branch called `issue/<issue number>`. One or more test cases to verify the bug is required.
+
+## Coding standards
+
+Simply use the same coding standard already found in the PHP files in the project.