Add CONTRIBUTING.md file

CONTRIBUTING.md

+# How to contribute
+
+## Preparing the development environment
+
+Make sure you have development dependencies installed in your development environment.
+
+```
+pip install -r requirements-dev.txt
+```
+
+We use `pre-commit` to maintain and install pre-commit hooks that should be executed
+before each commit. Use the following commands to install the hooks
+
+```
+pre-commit install
+```
+
+Running the pre-commit for the first time usually takes a little longer because `pre-commit`
+pulls some of it's hooks from upstream repositories.
+
+
+## Adding dependencies
+
+Make sure you update our `requirements.txt` file before you push your changes.
+Whenever you need a new python package, add it to requirements.in and run
+
+`pip-compile`
+
+to generate an new `requirements.txt` which does not only pin the new package
+but also its dependencies.
+
+If the new package you are adding is only used by developers,
+please add it to the `requirements-dev.txt` file.