add description for calendar & contacts category

- https://github.com/awesome-selfhosted/awesome-selfhosted/issues/2984

README.md

@@ -265,6 +265,8 @@ Software which allows users to add, annotate, edit, and share [bookmarks](https:
 
 **[`^        back to top        ^`](#)**
 
+[Electronic calendar](https://en.wikipedia.org/wiki/Calendaring_software) and [address book](https://en.wikipedia.org/wiki/Address_book)/[contact management](https://en.wikipedia.org/wiki/Contact_manager) software.
+
 _Related: [Groupware](#groupware)_
 
 _See also: [Comparison of CalDAV and CardDAV implementations - Wikipedia](https://en.wikipedia.org/wiki/Comparison_of_CalDAV_and_CardDAV_implementations)_