Document organizer behavior in README

Since the event organizer is the calendar owner's email
address and the link parameter is display information,
the README explains both roles and the CalDAV
scheduling constraint, including the profile email
requirement for working invitations.

README.md

@@ -41,11 +41,23 @@ the event and submits the form. The event lands in the user's
 | `description` | no | max 10000 chars |
 | `location` | no | HTTPS URL |
 | `url` | no | HTTPS URL; merged into description |
-| `organizer` | no | email address |
+| `organizer` | no | email address; display info only |
 
 Dates carry no timezone and are interpreted in the user's Nextcloud
 timezone (server default as fallback).
 
+## Organizer
+
+The event's ICS `ORGANIZER` is the Nextcloud user's own email
+address. Invitations to attendees are scheduled by the CalDAV
+backend in the calendar owner's name, which only works when the
+organizer matches the owner.
+
+The `organizer` link parameter is shown on the confirm page as
+meeting information and is used as `ORGANIZER` fallback only when
+the user has no profile email — in that case no working
+invitations may be sent, so profile emails should be set.
+
 ## Reminder
 
 Every created event gets a display alarm 15 minutes before it starts