Create CONTRIBUTING.md (#17)

Author: slavcodev

CONTRIBUTING.md

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+# Contributing guide
+
+The contribution is accepted via Pull Requests
+
+## Pull Requests
+
+- **Follow the conventions used in the project.** - Fix the code style with `$ php-cs-fixer fix`.
+
+- **Add tests!** - Your patch won't be accepted if it doesn't have tests.
+
+- **Document any change in behaviour** - Make sure the `README.md` and any other relevant documentation are kept up-to-date.
+
+- **Consider our release cycle** - We try to follow [SemVer v2.0.0](http://semver.org/).
+    Randomly breaking public APIs is not an option.
+
+- **Create feature branches** - Don't ask us to pull from your master branch, create branch, e.g. `feature-name` or `bugfix-title`.
+
+- **One pull request per feature** - If you want to do more than one thing, send multiple pull requests. Use appropriate labels.
+
+- **Send coherent history** - Make sure each individual commit in your pull request is meaningful.
+    If you had to make multiple intermediate commits while developing, please [squash them](http://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
+    Please read [How to write a good Git commit messages](https://chris.beams.io/posts/git-commit/)
+
+
+## Running Tests
+
+~~~bash
+$ phpunit
+~~~
+
+
+## Project's standards
+
+ * [PSR-1: Basic Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md)
+ * [PSR-2: Coding Style Guide](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)
+ * [PSR-4: Autoloading Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader.md)
+ * [PSR-5: PHPDoc (draft)](https://github.com/phpDocumentor/fig-standards/blob/master/proposed/phpdoc.md)
+ * Keep the order of class elements: static properties, instance properties, constructor (or setUp for PHPUnit), destructor (or tearDown for PHPUnit), static methods, instance methods, magic static methods, magic instance methods.
+
+
+**Enjoy coding**!