Add license file

[cxong]

Is your feature request related to a problem? Please describe. Please add a license file to this project.

• Make it easy to identify the license for this project - e.g. GitHub can scan a LICENSE file and include it in the header
• Better legal protection when a proper license is used (some jurisdictions do not recognise public domain)

Describe the solution you'd like I suggest Unlicense (https://choosealicense.com/licenses/unlicense/) or CC0 (https://creativecommons.org/share-your-work/public-domain/cc0/)

Copy the license text into a file at the project root like LICENSE, LICENSE.md, COPYING, or COPYING.md

Describe alternatives you've considered See above

Additional context See https://help.github.com/en/articles/licensing-a-repository for more info

[enriquesomolinos]

LICENSE-CC-BY-4.0.md was selected