Add license

[MrPropre]

I think you should add a LICENSE file to this repository (and mjml-php) to make it truly open source. It also allows to make an agreement between you and your users so that they are free to use your library.

https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/licensing-a-repository

Thank you.

[qferr]

Hello @MrPropre. You're right! I'll do it this week. Thank you!