Create CONTRIBUTING.md

Author: nerdoc

CONTRIBUTING.md

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+Contributing
+============
+
+Everyone is welcome to contribute to this project.
+
+Rules are easy. You can
+
+* Write an issue - if you find something like a problem. The more exact, the better.
+* Write documentation - that's when you already know the code a bit.
+* Write code - maybe you need a feature pydifact isn't capable of? Add it and make a PR.
+
+Writing code
+------------
+
+All files must be formatted using [Black](https://black.readthedocs.io). Easier is not possible.
+For bigger features it is recommended to create an own branch.