首先是安装：yarn global add create-react-app
create-react-app是类似vue-cli一样的脚手架，新手推荐
安装完之后开始创建第一个app吧！
采用方方的指令create-react-app react-demo-1会报错，如下：
A template was not provided. This is likely because you're using an outdated version of create-react-app.
Please note that global installs of create-react-app are no longer supported.
原因尚未可知。
于是跑去create-react-app的github瞅了一下（github.com/facebook/cr…），如下：
于是使用yarn create react-app react-demo3创建成功：
根据提示运行：
cd react-demo-3
yarn start得：
完。
