README: improve description

README.md

 Castellum Scheduler -- an open source tool for scheduling appointments
 
-Scheduler allows a group of people to coordinate so that everyone gets
-at most one of the available time slots. In a way this is the opposite
-of doodle where everyone agrees on the same time slot.
+Scheduler allows users to set up a set of timeslots and generate unique
+invitation links. Each invited person can then pick a free slot without
+needing to log in. In a way this is the opposite of doodle where
+everyone agrees on the same time slot.
 
 Scheduler is part of the [Castellum][1] project, but can be used with
 other tools just as well.