I'm working with Yarn v0.16.1. If I understand correctly (according to the documentation),
yarn global add <package>
npm install -g <package>
$ yarn global add create-react-app
$ yarn global v0.16.1
you should add
export PATH="$PATH:$(yarn global bin)" to your
~/.bash_profile or whatever you use, it would solve the issue.
depending on how you installed it, global folder varies for some reason.