Add a section about iCalendar to the manual

Fixes https://github.com/djcb/mu/issues/1825

mu4e/mu4e.texi

@@ -3204,6 +3204,7 @@ with other tools.
 * Org-mode links::Adding mu4e to your organized life
 * Org-contacts::Hooking up with org-contacts
 * BBDB::Hooking up with the Insidious Big Brother Database
+* iCalendar::Replying to iCal events
 * Sauron::Getting new mail notifications with Sauron
 * Speedbar::A special frame with your folders
 * Dired:: Attaching files using @t{dired}
@@ -3357,6 +3358,47 @@ After this, you should be able to:
 @item View the BBDB contact while viewing a message
 @end itemize
 
+@node iCalendar
+@section iCalendar
+
+When Gnus' article-mode is chosen (@ref{Message view}), it is possible
+to view and reply to iCalendar events.  To enable this feature, add
+
+@lisp
+(require 'mu4e-icalendar)
+(mu4e-icalendar-setup)
+@end lisp
+
+to your configuration.  If you want that the original invitation message
+be automatically trashed after sending the message created by clicking
+on the buttons “Accept”, “Tentative”, or “Decline”, also add:
+
+@lisp
+(setq mu4e-icalendar-trash-after-reply t)
+@end lisp
+
+When you reply to an iCal event, a line may be automatically added to
+the diary file of your choice.  You can specify that file with
+
+@lisp
+(setq mu4e-icalendar-diary-file "/path/to/your/diary")
+@end lisp
+
+Note that, if the specified file is not your main diary file, add
+@t{#include "/path/to/your/diary"} to you main diary file to display
+the events.
+
+To enable optional iCalendar→Org sync functionality, add the following:
+
+@lisp
+(setq gnus-icalendar-org-capture-file "~/org/notes.org")
+(setq gnus-icalendar-org-capture-headline '("Calendar"))
+(gnus-icalendar-org-setup)
+@end lisp
+
+Both the capture file and the headline(s) inside it must already exist.
+
+
 @node Sauron
 @section Sauron