Add information for the webcal and webcals protocols for subscribing to iCalendars

[sussexrick]

Safari doesn't seem to support webcals:// although other browsers do.

[cweiske]

• Firefox 96.0 on Ubuntu 20.04 does not handle webcal:// either (Nothing happens when clicking on such links).
• Chromium 97.0.4692.99 on Ubuntu 20.04 wants to pass that URL to the "xdg-open" command line tool when clicking on the link, and when pasting the link in the address bar google search is opened.