Instructions to decouple yarn from node generate error

[linc]

The docs state:

# Make sure not to link these or a yarn upgrade will upgrade your node!
brew install yarn --without-node

When I run that command, yarn reinstalls and generates this error:

Error: invalid option: --without-node

[charrondev]

Blame the brew maintainers. yarnpkg/website#913

This flag isn't supported anymore. The reasoning for it was that many junior devs don't know how to manage their $PATH env variable, and brew sucks! Yarn will prompt them to upgrade yarn, and then doing that through brew will upgrade node to whatever bleeding edge version is out. Then you end up with a bunch of broken versions of node-sass.

We should replace the instruction with

curl -o- -L https://yarnpkg.com/install.sh | bash

Yarn should not be managed through brew. It just causes issues.