Specify LICENSE

[mondeja]

As you can see in the README of simple-icons repository, the license badge of ngx-simple-icons third party extension is showing "not specified". You can resolve it just adding a LICENSE file to this repository.

Feel free to use whatever you want as the license only would refer to the code you are using to build the extension and not the icons you are serving, which are licensed under CC0 1.0 Universal.