Add a LICENSE file to the repo

[christopher-francisco]

After a merger, the vim guild in my company find ourselves in need to procure many different open source projects in order to use them. The process require use to submit a link to a license file.

I appreciate many of the neoclide/coc-* projects declares it's licensed as MIT in the README or package.json. Unfortunately we need to provide a link to a license file with the license contents so it's reviewed by legal. Without this, we won't be able to use the tools. I humbly request your support.

I have taken the liberty of opening #454 to address this.

I hope you consider this. Thanks in advance!

[chemzqm]

#454