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

[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!

[stoutmusclenugget]

@gaearon I would be more than happy to work on this.

[gaearon]

Sounds good!

[mobinni]

Oh, I forgot to comment. I actually just implemented this. However, since I didn't actually call dibs I would understand if @SMLMRKHLMS would take this on. It's in PR.

[stoutmusclenugget]

@mobinni Go for it! I haven't started yet :)

[mobinni]

@SMLMRKHLMS Thanks! Sorry man, I should have been more mindful I just started and forgot to comment.

[gaearon]

Fixed by #1193.