Document Yarn version

[eric-burel]

Related to #938 , we should update this part of the doc:

2. Install the dependencies. Make sure that you use Yarn 1.0.0 or higher.

yarn

You can use NPM instead of Yarn to handle the starter kit dependencies and to run scripts. Throughout the Apollo
Universal Starter Kit documentation, we'll always use Yarn.

Yarn 1.12.x won't work, so we could suggest to avoid it. Also I am not sure about saying you can use NPM instead of yarn.
It worked for adding packages in specific app with npm install in packages/server or packages/client, however to upgrade a package (eg extract-files to version 5.0.0 for Storybook to work), I've had to run yarn upgrade from the root of the project, otherwise it wouldn't update the yarn.lock.

Basically that's a tripping point for people not used to yarn especially in a boilerplate with multiple environment, so I think it's worth it to give a bit more details about yarn usage in the Readme or in other part of the doc.

[larixer]

@eric-burel Thank you Eric, good point! Yes, we no longer support npm, because we use Yarn workspaces

[eric-burel]

Also some issue I have is that core-js gets installed and added to the lock with version 1.0.0 whenever I use yarn add in packages/server. There might be one package with bad dependencies but I can't tell which. I have to remove it from the yarn.lock afterward and rerun yarn.