replace npm with yarn in gett started

[iezer]

Fixes #2696

[stefanpenner]

@iezer we are totally pushing towards this being possible, and working with (and some of us are on) the yarn team.

Currently the known blocker is: yarnpkg/rfcs#1

I suspect other details may surface as well.
One obvious downside will be: our getting started steps being:

npm i -g yarnpkg
yarn global add ember-cli
ember s

[rwjblue]

I suspect that we have to support both things either way (even if we internally use and suggest Yarn, I can't imagine ember-cli deciding to not support npm also)

[stefanpenner]

related issue on CLI: ember-cli/ember-cli#6341