Created by: fson
Yarn is a new fast, reliable and secure alternative to the npm client.
This pull request adds support for Yarn in Create React App. See discussion in #896 (closed).
In the create-react-app command, we try to install packages using Yarn. If Yarn is not installed, we use npm instead. In react-scripts, we detect if the project is using Yarn by checking if a yarn.lock file exists. If the project is using Yarn, all the instructions are displayed with Yarn commands and Yarn is used to install packages in init and eject scripts.