Add information on PRs to contribution guidelines. (#151)

doc/contributing.rst

@@ -2,10 +2,18 @@
 Contributing
 #######################
 
-We strongly welcome anyone interested in contributing to this project,
-be it with new ideas, suggestions, by filing bug reports or contributing code.
+We happily welcome anyone interested in contributing to this project,
+be it with new ideas, suggestions, by filing bug reports or contributing code
+to our `GitHub repository <https://github.com/PyPSA/PyPSA-Eur>`_.
 
-You are invited to submit pull requests and file issues to the `GitHub repository <https://github.com/PyPSA/PyPSA-Eur>`_.
+* If you already have some code changes, you can submit them directly as a pull request.
+* If you start working on a feature in the code, let us know by opening an issue or a draft pull request.
+  This helps all of us to keep an overview on what is being done and helps to avoid a situation where we
+  are doing the same work twice in parallel.
+* We encourage you to use the `PEP 8 coding style <https://www.python.org/dev/peps/pep-0008/>`_ in your code.
+
+For all code contributions we follow the four eyes principle (two person principle), i.e. all suggested code
+including our own are reviewed by a second person before they are incoporated into our repository.
 
 If you are unfamiliar with pull requests, the GitHub help pages have a nice `guide <https://help.github.com/en/articles/about-pull-requests>`_.