[gnome-shell] extensions-app: Add flathub badge to README

[Florian Müllner <fmuellner src gnome org>]

commit e2c1bfbedd6745e7d6c88f64436feaba28c4f82e
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Mar 31 23:58:45 2020 +0200

    extensions-app: Add flathub badge to README
    
    The app is now on flathub, so brag about that.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1160

 subprojects/extensions-app/README.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/subprojects/extensions-app/README.md b/subprojects/extensions-app/README.md
index 4aeceea3d1..6c0feb6485 100644
--- a/subprojects/extensions-app/README.md
+++ b/subprojects/extensions-app/README.md
@@ -5,6 +5,14 @@ used as a stand-alone project as well.
 
 Bugs should be reported to the GNOME [bug tracking system][bug-tracker].
 
+## Installation
+If Extensions is not already installed on your GNOME system, we
+recommend getting it from [flathub].
+
+<a href='https://flathub.org/apps/details/org.gnome.Extensions'>
+  <img width='240' alt='Download on Flathub' src='https://flathub.org/assets/badges/flathub-badge-en.png'/>
+</a>
+
 ## Building
 Before the project can be built stand-alone, the po directory has
 to be populated with translations (from gnome-shell).
@@ -20,4 +28,5 @@ License, version 2 or later. See the [COPYING][license] file for details.
 
 [logo]: logo.png
 [bug-tracker]: https://gitlab.gnome.org/GNOME/gnome-shell/issues
+[flathub]: https://flathub.org
 [license]: COPYING