[TASK] Rework CONTRIBUTING.md and move it to .github/

.github/CONTRIBUTING.md

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+# Contributing to this project
+
+:+1::tada: Many thanks for taking the time to contribute! :tada::+1:
+
+When you contribute, please take the following things into account:
+
+
+## Contributor Code of Conduct
+
+Please note that this project is released with a
+[Contributor Code of Conduct](../CODE_OF_CONDUCT.md). By participating in this
+project, you agree to abide by its terms.
+
+
+## Reporting an issue
+
+We have provided an issue template that will help you create helpful tickets.
+
+
+## Do not report an issue if …
+
+* … you are asking how to use some feature. Please use
+  [the phpList community](https://www.phplist.org/users/) for this purpose.
+* … your issue is about a security vulnerability. Please
+  [contact us directly](mailto:info@phplist.com) to report security issues.
+
+
+## Avoid duplicated issues
+
+Before you report an issue, please search through the existing issues here on
+GitHub to see if your issue is already reported or fixed to make sure you are
+not reporting a duplicated issue.
+
+Also please make sure you have the latest version of this package and check if
+the issue still exists.
+
+
+# Contribute code, bug fixes or documentation (pull requests)
+
+Third-party contributions are essential for keeping the project great.
+
+We want to keep it as easy as possible to contribute changes that get things
+working in your environment.
+
+There are a few guidelines that we need contributors to follow so that we can
+have a chance of keeping on top of things:
+
+1. Make sure you have a [GitHub account](https://github.com/join).
+2. [Fork this Git repository](https://guides.github.com/activities/forking/).
+3. Clone your forked repository and install the development dependencies doing
+   a `composer install`.
+4. Add a local remote "upstream" so you will be able to
+   [synchronize your fork with the original repository](https://help.github.com/articles/syncing-a-fork/).
+5. Create a local branch for your changes.
+6. Add unit tests for your changes (if your changes are code-related).
+   These tests should fail without your changes.
+7. Add your changes. Your added unit tests now should pass, and no other tests
+   should be broken. Check that your changes follow the
+   [coding style](#coding-style).
+8. Add a changelog entry.
+9. [Commit](#git-commits) and push your changes.
+10. [Create a pull request](https://help.github.com/articles/about-pull-requests/)
+    for your changes. Check that the Travis build is green. (If it is not, fix the
+    problems listed by Travis.)
+    We have provided a template for pull requests as well.
+11. [Request a review](https://help.github.com/articles/about-pull-request-reviews/).
+11. Together with your reviewer, polish your changes until they are ready to be
+    merged.
+
+
+## Unit-test your changes
+
+Please cover all changes with unit tests and make sure that your code does not
+break any existing tests. We will only merge pull request that include full
+code coverage of the fixed bugs and the new features.
+
+To run the existing PHPUnit tests, run this command:
+
+    vendor/bin/phpunit Tests/
+
+
+## Coding Style
+
+Please use the same coding style ([PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md))
+as the rest of the code. Indentation for all files is four spaces.
+
+We will only merge pull requests that follow the project's coding style.
+
+Please check your code with the provided PHP_CodeSniffer standard:
+
+    vendor/bin/phpcs --standard=PSR2 Classes/ Tests/
+
+Please make your code clean, well-readable and easy to understand.