Clarify that mkcert and be used as an infrastructure CA

Filippo, as we discussed as GothamGo, using `mkcert` as an infrastructure CA to secure internal service-to-service http calls is probably a use case falls within the mission of `mkcert`. The readme provides enough detail on how to do this manually, but it warns against "production" use. Please clarify that this use case is not discouraged. Thanks.