SPF: explain what should the zone have in order to work.

src/App/Text/Explanations.purs

@@ -98,6 +98,7 @@ spf_introduction =
     [ HH.u_ [HH.text "Advice for novice users"]
     , HH.text """
       : default values should work great with simple domains.
+      Make sure to have a MX record, which should be pointing to an A or AAAA record, and that will do it. 🥳
       """
     ]
   ]