Update contribution guidelines

txt requirements files were replaced by sections in setup.py

fixes ihabunek#457

docs/contributing.md

@@ -80,9 +80,7 @@ source _env/bin/activate
 # On Windows
 _env\bin\activate.bat
 
-pip install --editable .
-pip install -r requirements-dev.txt
-pip install -r requirements-test.txt
+pip install --editable ".[dev,test]"
 ```
 
 While the virtual env is active, running `toot` will execute the one you checked