Add yarn example command

[thangman22]

trafficstars

We recommend user to use Yarn but Document is not available yet. That reason why I put this on this PR

[jeffposnick]

Thanks for your contribution! Most of these changes apply to documentation that was originally copied over from https://github.com/vuejs-templates/webpack

Rather than have the docs diverge further, I'd ask that you submit an equivalent PR against https://github.com/vuejs-templates/webpack, and go through the review process there. If it's merged, we can make the same changes to this project's docs.