Update docs to explain primary_email_field

[francoismajor]

Hi! I have different forms that use different handles for the email field, and the subscribe wasn't working. I found the primary_email_field in the code and defined it in my config and now my forms work. Maybe add that as an option in the docs? (and in the published config file)

/*
  * Define the handle for the email field to be used. Defaults to email.
  */
'primary_email_field' => 'email',

Thanks!

[edalzell]

Thanks for this, updated the readme and the config