Informieren is in fact in all cases a transitive verb. The second most common use is probably reflexive, when someone acquires certain knowledge for himself.
You may encounter some rare cases of usage without an object (note this doesn't necessarily mean the verb is intransitive), as in your example. This is when the object (who was informed) is implicitely obvious, as in your example (the reader) or in
Die Tagesschau informiert täglich mehrmals über das Weltgeschehen
Der Regierungssprecher informierte über die aktuellen Vorgänge im Kabinett