Added note on about black in documentation

docs/source/CONTRIBUTE.md

@@ -90,6 +90,15 @@ the core development team.
 * If you're adding code that should be tested, add tests
 * If you're adding or modifying examples, make sure to add them to `test_examples.py`
 
+### Formatting code
+
+* MALA uses `black` for code for unified code formatting
+  * For more info on `black` itself, see the respective 
+    [documentation](https://github.com/psf/black)
+* Currently, no automatic code reformatting will be done in the CI, thus
+  please ensure that your code is properly formatted before creating a pull 
+  request
+
 ### Adding dependencies
 
 If you add additional dependencies, make sure to add them to `requirements.txt`
@@ -98,7 +107,6 @@ they are not.
 Further, in order for them to be available during the CI tests, make sure to 
 add _required_ dependencies to the appropriate environment files in folder `install/` and _extra_ requirements directly in the `Dockerfile` for the `conda` environment build.
 
-
 ## Pull Requests
 We actively welcome pull requests.
 1. Fork the repo and create your branch from `develop`