Add documentation and configuration for mjml

[Hultner]

There's currently no information of how to updated and build new templates from the mjml-files, nor is there any information on what config or options was used to generate the original files. Simply using the minify option doesn't yield the same results.

[mariusenloya]

Please can someone help with this issue? @tiangolo

[karolzlot]

Solution is in https://github.com/tiangolo/full-stack-fastapi-postgresql/issues/141