Add note about isort

CONTRIBUTING.md

@@ -29,10 +29,11 @@ with your idea or suggestion and we'd love to discuss about it.
 
 - Fork the repo on [GitHub](https://github.com/GeoStat-Framework/GSTools)
 - Add yourself to AUTHORS.md (if you want to).
-- We use the black code format, please use the script `black .` after you have written your code.
+- We use [black](https://github.com/psf/black) and [isort](https://github.com/PyCQA/isort) to format our code.
+  Please use the scripts `black .` and `isort .` after you have written your code.
 - Add some tests if possible.
 - Add an example showing your new feature in one of the examples sub-folders if possible.
-  Follow this [Sphinx-Gallary guide](https://sphinx-gallery.github.io/stable/syntax.html#embed-rst-in-your-example-python-files)
+  Follow this [Sphinx-Gallary guide](https://sphinx-gallery.github.io/stable/syntax.html#embed-rst-in-your-example-python-files).
 - Push to your fork and submit a pull request.
 
 ### PyLint Settings
@@ -91,4 +92,4 @@ Here is a list of the occurring disabled errors:
   (invalid-unary-operand-type) - false positive at some points
 
 Although we disabled these errors at some points, we encourage you to prevent
-disabling errors when it is possible.
+disabling errors when it is possible.