Add CONTRIBUTING.md

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diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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@@ -0,0 +1,10 @@
+# Contributing
+
+## Code style
+
+* Make sure your code follows the [PSR-12](https://www.php-fig.org/psr/psr-12/) code style.
+You may use `composer run phpcs` to verify that.
+* Use `use` statements wherever possible instead of writing the fully qualified name.
+* Order the composer/npm dependencies alphabetically.
+* Do not use code from the [includes](./includes) directory anywhere else.
+* If possible cover your code by test cases.
diff --git a/README.md b/README.md
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--- a/README.md
+++ b/README.md
@@ -87,6 +87,8 @@ Since the engelsystem is open source, you can help to improve the system. We rea
 
 Please create single pull requests for every feature instead of creating one big monster of pull request containing a complete rewrite.
 
+Please read the [CONTRIBUTING.md](./CONTRIBUTING.md) before you start.
+
 ### Testing
 To run the unit tests use
 ```bash