[devhelp] readme: add flatpak install section

[Sébastien Wilmet <swilmet src gnome org>]

commit d000f5668cd1dbf67dbef299426ccb60da922488
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Apr 26 20:40:02 2020 +0200

    readme: add flatpak install section

 README.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/README.md b/README.md
index 81c01b68..d6a85e64 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,19 @@ Professional services
 
 See the file: [docs/professional-services.md](docs/professional-services.md)
 
+Installation of the Devhelp Flatpak
+-----------------------------------
+
+### Stable version
+
+- [Devhelp on Flathub](https://flathub.org/apps/details/org.gnome.Devhelp)
+
+### Nightly, development version
+
+```
+$ flatpak install https://gitlab.gnome.org/GNOME/gnome-apps-nightly/raw/master/devhelp.flatpakref
+```
+
 How to contribute
 -----------------