[devhelp] HACKING file: add Flatpak section

[Sébastien Wilmet <swilmet src gnome org>]

commit e06dbeca60ed0af23728e358c1555f64e3cc739d
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Fri Jun 15 16:35:19 2018 +0200

    HACKING file: add Flatpak section

 HACKING | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/HACKING b/HACKING
index fc16ff23..8e5e36c2 100644
--- a/HACKING
+++ b/HACKING
@@ -27,3 +27,15 @@ https://blogs.gnome.org/swilmet/2015/10/10/changing-quickly-between-a-final-and-
 
 See also:
 https://developer.gnome.org/programming-guidelines/unstable/
+
+Flatpak
+-------
+
+The Devhelp maintainers maintain both the nightly and the stable versions of
+the Devhelp Flatpak.
+
+The nightly version is in flatpak/ and the gnome-apps-nightly module:
+https://gitlab.gnome.org/GNOME/gnome-apps-nightly
+
+The stable version is on Flathub:
+https://github.com/flathub/org.gnome.Devhelp