[gimp] build: update the flatpak-howto.

[Jehan Pagès <jehanp src gnome org>]

commit 590924f85699c9259c63421fa317446e720698d1
Author: Jehan <jehan girinstud io>
Date:   Fri May 12 21:40:43 2017 +0200

    build: update the flatpak-howto.
    
    In particular, I didn't have the correct metadata because I was missing
    appstream-compose. This should be a dependency of flatpak. But for the
    time being, let's just have a note in our howto.

 build/flatpak/flatpak-howto.txt |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/build/flatpak/flatpak-howto.txt b/build/flatpak/flatpak-howto.txt
index dd01307..3d3d572 100644
--- a/build/flatpak/flatpak-howto.txt
+++ b/build/flatpak/flatpak-howto.txt
@@ -1,6 +1,14 @@
 GIMP Flatpak HowTo
 ==================
 
+* Install flatpak and flatpak-builder. There are packages for most
+  distributions. Check: http://flatpak.org/getting.html
+
+* Make sure `appstream-compose` is installed as well. This is used to parse the
+  appdata file and generate the appstream (metadata like comments, etc.).
+  For instance on Fedora, this is provided by the package `libappstream-glib`,
+  on Ubuntu by `appstream-util`…
+
 * GIMP uses Flatpak's GNOME runtime, which contains a base of libraries,
   some of which are dependencies of GIMP.
   Check out the available versions at: http://flatpak.org/runtimes.html