[libnotify] README: Add reference to Portal Notification APIs

[Marco Trevisan <marcotrevi src gnome org>]

commit 5e6a52fa4fe880c0b3ef9b9360bb8d89f113c90f
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Tue Jun 7 02:46:08 2022 +0200

    README: Add reference to Portal Notification APIs

 README.md | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/README.md b/README.md
index f9ebf9e..219d209 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,9 @@ daemon, as defined in the [org.freedesktop.Notifications][fdo-spec] Desktop
 Specification. These notifications can be used to inform the user about an event
 or display some form of information without getting in the user's way.
 
+It is also a simple wrapper to send cross-desktop Notifications for sandboxed
+applications using the [XDG Portal Notification API][portal].
+
 ## Notice
 
 For GLib based applications the [GNotification][gnotif] API should be used
@@ -14,3 +17,4 @@ instead.
 
 [fdo-spec]: https://specifications.freedesktop.org/notification-spec/notification-spec-latest.html
 [gnotif]: https://docs.gtk.org/gio/class.Notification.html
+[portal]: 
https://github.com/flatpak/xdg-desktop-portal/blob/main/data/org.freedesktop.portal.Notification.xml