Document using yarn for getting started in readme

Created by: bondz

Yarn has support for using create-react-app without first installing the binary via

yarn create react-app app-name

I understand most beginners might not have yarn installed, but it can help avoid issues like #1965 (closed). Yarn is stable and some people would have it on their machine.

I'd have created a PR but I want to understand why it wasn't documented earlier. I've mostly used this option when creating CRA apps, its mostly faster than npm too.

If someone ends up using yarn to install modules after creating an app using npx which currently defaults to npm, yarn does an integrity check and reinstalls the tree. We already document that npm >= 5 does the same thing.