Create CONTRIBUTING.md

CONTRIBUTING.md

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+## Contribution ##
+
+citeproc-php is an Open Source project. You can support it by reporting bugs, contributing code or contributing documentation.
+
+### Star the Repo ###
+Developing software is a hard job and one has to spend a lot of time. Every open-source developer is looking forward 
+about esteem for his work. If you use citeproc-php and if you like it, star it and talk about it in Blogs.
+
+### Reporting a Bug ###
+Use the [Issue Tracker](https://github.com/seboettg/citeproc-php/issues) in order to report a bug.
+
+### Contribute Code ###
+You are a developer and you like to help developing new features or bug fixes? Fork citeproc-php, setup a workspace and send
+a pull request.
+
+I would suggest the following way:
+
+* Fork citeproc-php on Github
+* Clone the forked repo
+```bash
+$ git clone https://github.com/<yourname>/citeproc-php
+``` 
+* Setup your preferred IDE
+* Run the UnitTests within your IDE
+* Write a test case for your issue. My tests are based on the original [test-suite](https://github.com/citation-style-language/test-suite). You can build custom (human-readable) test cases following the described [Fixture layout](https://github.com/citation-style-language/test-suite#fixture-layout). 
+* Additionally, you have to translate (human-readable) test-cases into json format (machine-readable)
+```bash
+$ cd <project-root>/tests/fixtures/basic-tests
+$ ./processor.py -g
+```
+* create a test function within an already existing test class or create a new test class:
+```php
+<?php 
+namespace Seboettg\CiteProc;
+use PHPUnit\Framework\TestCase;
+
+class MyNewClassTest extends TestCase
+{
+    use TestSuiteTestCaseTrait;
+    // ...
+    public function testMyBrandNewFunction() 
+    {
+        //my brand new function is the file name (without file extension)
+        $this->_testRenderTestSuite("myBrandNewFunction");
+    }
+    // ...
+}
+```
+* Make sure that your test case covers relevant code parts
+* Implement the code until all tests finishing successfully
+* Send a pull request