Add pre-commit flake8/isort checks (#6058)

After setting up the pre-commit hooks you will get `flake8` and `isort` checks.

Co-authored-by: Armen Zambrano <armenzg@users.noreply.github.com>

.pre-commit-config.yaml

@@ -0,0 +1,9 @@
+repos:
+  - repo: https://gitlab.com/pycqa/flake8
+    rev: 3.7.9
+    hooks:
+      - id: flake8
+  - repo: https://github.com/timothycrosley/isort
+    rev: 4.3.21
+    hooks:
+      - id: isort

docs/installation.md

@@ -12,6 +12,14 @@ If you only want to hack on the frontend, see the UI Development section below.
 
 Before pushing new code, please make sure you are following our [Code Style](code_style.md#ui) and [Accessibility Guidelines](accessibility.md).
 
+- If you would like pre-commit linting checks you can set it up like this:
+  ```console
+  % pip install pre-commit
+  % pre-commit install
+  pre-commit installed at .git/hooks/pre-commit
+  ````
+  From here on, linting checks will be executed every time you commit.
+
 ## UI Development
 
 To get started: