added instruction for error running 'python setup.py.docs'

Author: yohanmoon

docs/README.md

@@ -11,6 +11,7 @@ Steps:
 - Install Sphinx (`pip install Sphinx`)
 - Fork the pygame-ce repository, download and navigate to it in the terminal
 - Run `python setup.py docs`
+     - (Run `python -m pip install -U pip setuptools` first if `ModuleNotFoundError: No module named setuptool` occurs)
 
 This will create a new folder under the `docs` folder. 
 In `docs/generated`, you will find a local copy of the pygame documentation.