Update CONTRIBUTING guide

Author: timvink

CONTRIBUTING.md

@@ -8,7 +8,7 @@ Thanks for considering to contribute to this project! Some guidelines:
 - This package tries to be as simple as possible for the user (hide any complexity from the user). Options are only added when there is clear value to the majority of users.
 - When issues or pull requests are not going to be resolved or merged, they should be closed as soon as possible. This is kinder than deciding this after a long period. Our issue tracker should reflect work to be done.
 
-## Testing
+## Unit Tests
 
 Make sure to install an editable version before running tests:
 
@@ -27,9 +27,18 @@ pip install pyflakes
 pyflakes tests/ mkdocs_git_authors_plugin/
 ```
 
-#### Code Style
+## Manual testing
 
-Make sure your code *roughly* follows [PEP-8](https://www.python.org/dev/peps/pep-0008/) and keeps things consistent with the rest of the code.
+To quickly serve a website with your latest changes to the plugin use the sites in our tests suite. For example:
 
-We use google-style docstrings.
+```python
+pip install -r tests/test_requirements.txt
+pip install -e .
+mkdocs serve -f ./tests/basic_setup/mkdocs_complete_material.yml
+```
+
+## Code Style
+
+Make sure your code *roughly* follows [PEP-8](https://www.python.org/dev/peps/pep-0008/) and keeps things consistent with the rest of the code. I recommended using [black](https://github.com/psf/black) to automatically format your code.
 
+We use google-style docstrings.