org-contacts.el: Add usage comments

* contrib/lisp/org-contacts.el: Add usage comments.

contrib/lisp/org-contacts.el

@@ -50,7 +50,14 @@
 ;; :ADDRESS:
 ;; :BIRTHDAY:
 ;; :END:")))
-;;
+
+;;;; Usage:
+
+;;; How to search?
+;;; 
+;;; You can use `org-spare-tree' [C-c / p] to filter based on a
+;;; specific property. Or other matcher on `org-sparse-tree'.
+
 ;;; Code:
 
 (require 'cl-lib)