Notify that Smaily integration won't work if form fails sending mail

Every time I set up the dev environment and test the plugin out by filling out the form it pops up an error message. Initially puzzled for a few sec (open up dev tools) and then realize I need to enable additional settings (either demo_mode or skip_mail) for the form as I've not configured mail sending.

Describe the solution you'd like
User is notified that contacts are added to Smaily only if CF7 ends successfully. If mail settings are unconfigured and skip_mail: on is missing in form additional settings, the integration won't work with the form.

Describe alternatives you've considered
Adding it to the README

Additional context
Setting up mail is a no-brainer if you have used Contact Form 7 before but I think we should still mention this somewhere as most of our instructions are hammered down and to the point.