Adds flake8 and fixes style issues

CONTRIBUTING.md

@@ -8,6 +8,15 @@ We use [black](https://github.com/psf/black) as our base code formatter with a l
 ```bash
 black --line-length=120 {source_file_or_directory}
 ```
+
+In additon, we use [flake8](https://flake8.pycqa.org/en/latest/) to detect style issues with the code:
+
+```bash
+flake8 --max-line-length=120 {source_file_or_directory}
+```
+
+ Not all outputs from flake8 are mandatory. For instance, splitting **bullet points in docstrings (E501)** will cause issues when generating the docuementation, so we will leave that longer than the line lenght limit . Another example are **whitespaces before colons in inline fors (E203)**. `black` places them in that way, so we'll leave them like that.
+
 On top of that, there are a few rules to also have in mind.
 
 ### Code Spacing