add content from #1073

DOCUMENTATION.md

@@ -1272,7 +1272,8 @@ Default: 10000
 Radicale has been tested with:
 
 * [Android](https://android.com/) with
-  [DAVx⁵](https://www.davx5.com/) (formerly DAVdroid)
+  [DAVx⁵](https://www.davx5.com/) (formerly DAVdroid),
+* [OneCalendar](https://www.onecalendar.nl/)
 * [GNOME Calendar](https://wiki.gnome.org/Apps/Calendar),
   [Contacts](https://wiki.gnome.org/Apps/Contacts) and
   [Evolution](https://wiki.gnome.org/Apps/Evolution)
@@ -1303,6 +1304,13 @@ Enter the URL of the Radicale server (e.g. `http://localhost:5232`) and your
 username. DAVx⁵ will show all existing calendars and address books and you
 can create new.
 
+#### OneCalendar
+
+When adding account, select CalDAV account type, then enter user name, password and the
+Radicale server (e.g. `https://yourdomain:5232`). OneCalendar will show all
+existing calendars and (FIXME: address books), you need to select which ones
+you want to see. OneCalendar supports many other server types too.
+
 #### GNOME Calendar, Contacts
 
 GNOME 46 added CalDAV and CardDAV support to _GNOME Online Accounts_.