[Documentation] Please consider adding one additional line to the README

[rubyFeedback]

mail has a super nice API. I like it.

The documentation currently reads:

Sending via smtp (for example to [mailcatcher](https://github.com/sj26/mailcatcher))

Could one sentence be added to briefly explain why one may want to use smtp?

Right now if you read the examples from top to bottom, and if you never used mail related stuff, you may not know what smtp is, and why you'd need it. Right now I wonder if I need it or not. It would be nice if the README can BRIEFLY explain why one may need smtp VERSUS why one may NOT need smtp. Right now I don't know and have to read up on SMTP externally but I'd like to have this briefly explained in the main README as-is.