[Documentation] --use-npm & --scripts-version

Created by: adeelibr

• I have checked the issues & could't find the mentioned below.

There should be mention of the following;

• --use-npm in the documentation, if someone want's to opt out of yarn to use npm instead.
• How to use --scripts-version when over-riding with your own custom react-scripts.

npx create-react-app --scripts-version CUSTOM_SCRIPT_PATH my-project-name --use-npm

I am more than happy to make a PR for this. (I would need some guideance though :))