Add a gentle nudge for using npm >= 3 or Yarn

Created by: gaearon

If you're using npm 2, I'd like us to print a message:

$ create-react-app hello
Creating a new React app in /home/ubuntu/workspace/hello.

Tip: It looks like you are using npm 2. We suggest using npm 3 or Yarn for faster install times and less disk space usage.

Installing packages. This might take a couple minutes.
Installing react-scripts.

(The "Tip" is the new part.)

A PR would be welcome!