Add a LICENSE file

Hi, nice project.

I deduced from the `package.json` that you are MIT-licensed. You could make this more obvious by adding a separate license file.

PS. Based on MIT license I can (and will) PR you to [delightful-databases](https://codeberg.org/yarmo/delightful-databases).

Start by reading package.json to confirm the stated MIT licensing. Add a separate LICENSE file containing the MIT license, then verify that the repository clearly exposes the license information.

索引モデルが issue の本文から書いたものです。