前言
在官方介绍里有这么一句话：
Yarn is a package manager for your code. It allows you to use and share code with other developers from around the world. Yarn does this quickly, securely, and reliably so you don’t ever have to worry.
关键意思就是，快速,安全,可靠。你下载的包将不再重新下载。而且
