Update README.md to steer users to Gtk4 (#699)

* Update README.md to steer users to Gtk4

I sure wish someone had done this earlier!

* Fix typos

README.md

@@ -6,6 +6,12 @@
 [![codecov.io](https://codecov.io/github/JuliaGraphics/Gtk.jl/coverage.svg?branch=master)](https://codecov.io/github/JuliaGraphics/Gtk.jl?branch=master)
 [![](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliagraphics.github.io/Gtk.jl/latest)
 
+This package is NO LONGER SUPPORTED.
+Users should migrate to
+[Gtk4.jl](https://github.com/JuliaGtk/Gtk4.jl),
+following the
+[tips on the differences betwen Gtk3 and Gtk4](https://juliagtk.github.io/Gtk4.jl/dev/diff3to4).
+
 GUI building, using the Gtk library: [https://www.gtk.org/](https://www.gtk.org/)
 
 Complete Gtk documentation is available at [https://www.gtk.org/docs](https://www.gtk.org/docs)