Add MIT license file to root directory

> [Vue 3 CheatSheet](https://vue-cheatsheet.themeselection.com/) is an open source project by [ThemeSelection](https://themeselection.com/) that is licensed under [MIT](http://opensource.org/licenses/MIT).

Would you consider adding a MIT license file to this repository's root directory? It would show up in GitHub search results as MIT licensed; better SEO.