Add yarn instructions

[dword-design]

Many people are using Yarn to build their projects, so it makes sense to also document this.

[coveralls]

Coverage remained the same at 93.75% when pulling 0786a78 on dword-design:2019-10-30/yarn into 9369b46 on mosjs:master.

[zkochan]

@dword-design excuse me but I don't have time to maintain this project. I am too busy with pnpm which is also a js package manager :-)

I recommend you to check out projectz it can also generate your readme files and it seems maintained.

There is also https://github.com/zeke/package-json-to-readme

[dword-design]

@zkochan great thanks. projectz seems to be my solution.