[gnome-control-center] docs: advertise the flatpak

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit a8a6dc0f882f0d2e2ffca4a905dac52f9ad71e3c
Author: Claudio André <claudioandre br gmail com>
Date:   Wed May 16 15:20:17 2018 -0300

    docs: advertise the flatpak
    
    [skip ci]

 README.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/README.md b/README.md
index f6b48a006..a3b48b9e4 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,15 @@ your desktop.
 See `docs/CONTRIBUTING.md` for details on the contribution process, and `docs/HACKING.md`
 for the coding style guidelines.
 
+## Testing Unstable Settings
+
+It is quite easy to test and give feedback about the development version of GNOME
+Settings. Just access https://gitlab.gnome.org/GNOME/gnome-control-center/environments,
+get the latest version, download it, double-click the file, install and run.
+
+Note that GNOME Settings Flatpak will only work if you are running
+the latest GNOME version in your host system.
+
 ## Reporting Bugs
 
 Bugs should be reported to the GNOME bug tracking system under the product