[fractal] readme: Add instructions for the nightly flatpak

[Julian Sparber <jsparber src gnome org>]

commit 6eeac2d0e6b1236570dc04b6ade5ca02a72c467a
Author: Kévin Commaille <zecakeh tedomum fr>
Date:   Thu Mar 31 11:46:21 2022 +0200

    readme: Add instructions for the nightly flatpak

 README.md | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/README.md b/README.md
index 912093d53..fa8528775 100644
--- a/README.md
+++ b/README.md
@@ -29,6 +29,8 @@ yet ready for a release and you can follow along our progress towards it by look
 
 ## Installation instructions
 
+### Stable version
+
 Flatpak is the recommended installation method. Until Fractal-next is ready, you can get the official
 Fractal Flatpak on Flathub.
 
@@ -42,6 +44,19 @@ Fractal can also be installed as a snap on any distro with snap support enabled
 <img src="https://github.com/snapcore/snap-store-badges/raw/master/EN/[EN]-snap-store-white.png"; 
width="182px" />
 </a>
 
+### Development version
+
+If you want to try Fractal-next without building it yourself, it is available as a nightly Flatpak
+in the gnome-nightly repo.
+
+```sh
+# Add the gnome-nightly repo
+flatpak remote-add --user --if-not-exists gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo
+
+# Install the nightly build
+flatpak install --user gnome-nightly org.gnome.Fractal.Devel
+```
+
 ## Build Instructions
 
 ### Flatpak