Updated CONTRIBUTING.md: flake8's default .venv folder excluding (#218)

* Updated CONTRIBUTING.md: flake8's default .venv folder excluding

* Updated CONTRIBUTING.md: flake8's default .venv folder excluding

Author: paveldroo

CONTRIBUTING.md

@@ -45,6 +45,8 @@ To run linting:
 ```bash
 flake8 .
 ```
+Keep in mind: default virtual environment folder excluded by flake8 style checking is `.venv`.
+If you want to customize this parameter, you should do this in `setup.cfg`.
 
 These steps are mandatory during the CI.