Add LICENSE file

[christopher-francisco]

The project's REDME says the license is MIT. I am just adding the file. Taken from https://choosealicense.com/licenses/mit/.

Why?

I've recently joined a big corporate environment and we need to go through an approval process for every single plugin. Having a link to the license file makes me able to go through that process. I'm doing this for every vim plugin I use.

Other LICENSE PRs I've raised:

• https://github.com/neoclide/coc-tsserver/pull/454
• https://github.com/neoclide/coc-eslint/pull/145
• https://github.com/neoclide/coc-prettier/pull/173