[gnome-shell-extensions] README: Re-add extension alternate-tab with obsolescence notice

[Florian Müllner <fmuellner src gnome org>]

commit d94a3500f64049b019f224bb54e1dedf642afd01
Author: Fabian P. Schmidt <kerel-fs gmx de>
Date:   Tue Mar 19 22:24:44 2019 +0100

    README: Re-add extension alternate-tab with obsolescence notice
    
    This commit adds a short notice about the former extension
    alternate-tab to the README. This notice should be removed once
    the traffic of users searching for this obsolete extension settled
    down.
    
    https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/62

 README.md | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/README.md b/README.md
index 1587a97..3b84cb1 100644
--- a/README.md
+++ b/README.md
@@ -15,6 +15,12 @@ Bugs should be reported to the GNOME [bug tracking system][bug-tracker].
 
 ## Extensions
 
+ * alternate-tab (**OBSOLETE**)
+ 
+     Lets you use classic Alt+Tab (window-based instead of app-based) in GNOME Shell.
+     This extension is obsolete since GNOME 3.30, see [this blogpost][alternatetab-post]
+     for further details.
+
  * apps-menu
 
      Lets you reach an application using gnome 2.x style menu on the panel.
@@ -74,3 +80,4 @@ file for details.
 [shell-page]: https://wiki.gnome.org/Projects/GnomeShell
 [bug-tracker]: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues
 [license]: COPYING
+[alternatetab-post]: 
https://blogs.gnome.org/fmuellner/2018/10/11/the-future-of-alternatetab-and-why-you-need-not-worry/