[gtk-mac-integration] Add notes about obsolesence and Gtk4 non-support to docs and README.

[John Ralls <jralls src gnome org>]

commit 42d5af404edad29f33bbc04f11f9d08ebeb2d087
Author: John Ralls <jralls ceridwen us>
Date:   Tue Mar 22 10:19:47 2022 -0700

    Add notes about obsolesence and Gtk4 non-support to docs and README.

 README                  |  4 +++-
 src/gtkosxapplication.c | 10 +++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/README b/README
index ea37e55..ac2a870 100644
--- a/README
+++ b/README
@@ -6,7 +6,9 @@ bundles. GtkosxApplication is gobject-introspectable.
 
 GtkosxApplication compiles and works with i386, x86_64 and arm64.  New
 Gtk3 applications should prefer the GApplication/GtkApplication and
-GMenuModel/GMenu APIs which make this library unnecessary.
+GMenuModel/GMenu APIs which make this library unnecessary. Gtk4
+applications must use those facilities as the older API on which this
+libary depends have been remobed.
 
 Example and test code may be found in src/test-integration.c; Being
 GUI code this library is somewhat resistant to useful automated
diff --git a/src/gtkosxapplication.c b/src/gtkosxapplication.c
index 628667c..b0f97f7 100644
--- a/src/gtkosxapplication.c
+++ b/src/gtkosxapplication.c
@@ -36,7 +36,15 @@
  * Exposes to the Gtk+ program important functions of
  * OS X's NSApplication class for use by Gtk+ applications running with
  * the quartz Gdk backend and provides addtional functions for
- * integrating a Gtk+ program into the OS X user environment.
+ * integrating a Gtk+ program into the macOS user environment.
+ *
+ * Note: There's a complete example program in src/testintegration.c.
+ *
+ * Note: This library is obsolescent in Gtk3 as there are built-in
+ * features in GLib and Gtk to integrate Gtk applications with macOS.
+ *
+ * Note: gtkosxapplication does not and cannot support Gtk4. Use the
+ * built-in macOS integration features.
  *
  * Using GtkosxApplication is pretty simple.
  * First, create an instance at startup: