The usefulness of this package is questionable: it should be pretty
trivial for a library user to wrap a backend instead of using this.

The GitHub UI already displays that information.

This was originally added to the README.md in 9e949e0, but was lost when
the examples were removed from there. Re-add it so that it's clearer
that this library doesn't enforce authentication by default.

Closes: #275

Closes: #189

Client.SendMail() already calls validateLine() via Client.Mail() and
Client.Rcpt(), so there is no need to have these checks in sendMail().

Having these checks don't hurt as such, but looking at just the
sendMail() function it's not obvious whether these checks are needed if
you want to implement your own SendMail()-type function. With this
change, you can copy/paste sendMail(), add the smtp. package selector,
and modify as needed.

Also removes a superfluous nil check as a bonus.