Add support for syncing TODOs

README.md

@@ -238,6 +238,11 @@ synced and which not:
   `org-caldav-skip-conditions`. The syntax of the conditions is
   described in the doc-string of `org-agenda-skip-if`.
 
+* In case you just want to keep your remote calendar clean, set
+  `org-caldav-days-in-past` to the number of days you want to keep in
+  the past on the remote calendar.  This does not affect your org files,
+  it works just as a filter for entries older than N days.
+
 Note however that the normal `org-agenda-skip-function(-global)` will
 **not** have any effect on the icalendar exporter (this used to be the
 case, but changed with the new exporters).
@@ -398,6 +403,38 @@ the events into the debug buffer. If you send such a buffer in a bug
 report, please make very sure you have removed personal information
 from those events.
 
+#### Syncing TODO
+
+This section is experimental.  So please make sure you have backups before you
+use this.  Create issues if it doesn't work for you.  This was developed with a
+nextcloud 12 server, and it is not sure if it works somewhere else.  Please
+report if you have another setup, so we can fix that.
+
+To enable this, just use
+
+    (setq org-icalendar-include-todo 'all
+        org-caldav-sync-todo t)
+
+The first will include your todos to the exporter (org->cal) the second controls
+the use of downloaded vtodo events to import it to the `inbox`.
+
+It is also a good idea to set
+
+    (setq org-icalendar-categories '(local-tags))
+
+so the tags on the nextcloud will not include the local category.  If you need
+that, leave a message.  On cal->org this would add it to the tags.
+
+See the variables `org-caldav-todo-priority` and
+`org-caldav-todo-percent-states` for some customization.
+
+There is also `org-caldav-todo-deadline-schedule-warning-days` which can be set
+to `t` if you want to include a scheduled date.  Some apps like OpenTasks will
+move tasks out of the widget if there is a scheduled, until it actually starts.
+
+I would recommend you to test this and report issues, because I also want to
+have a stable todo handling.
+
 #### Known Bugs
 
 * Recurring events created or changed on the calendar side cannot be
@@ -410,6 +447,11 @@ from those events.
 * Pretty much everything besides SUMMARY, DESCRIPTION, LOCATION and
   time is ignored in iCalendar.
 
+* When trying to sync todo entries org-mode's export to ical can create empty
+  entries.  org-caldav tries to sync this, which results in errors.
+  In this case you can set `org-icalender-sync-todo` to `t` instead of `'all`,
+  until it is fixed.
+
 #### How syncing happens (a.k.a. my little CalDAV rant)
 
 (This is probably not interesting, so you can just stop reading.)