[gimp] build: update flatpak howto.

[Jehan Pagès <jehanp src gnome org>]

commit c99800b1045d3bc61a12be642daf80a4842fd43f
Author: Jehan <jehan girinstud io>
Date:   Mon Jan 30 17:35:28 2017 +0100

    build: update flatpak howto.

 build/flatpak/flatpak-howto.txt |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/build/flatpak/flatpak-howto.txt b/build/flatpak/flatpak-howto.txt
index 7407dd9..94450c3 100644
--- a/build/flatpak/flatpak-howto.txt
+++ b/build/flatpak/flatpak-howto.txt
@@ -89,3 +89,9 @@ Or simply update them:
   `.flatpakrepo` file which we can upload on `gimp.org` and should allow
   GNU/Linux users to install GIMP and our flatpak repo in a few clicks.
   We will also have to sign the repo with GPG, etc.
+
+* TODO: Windows builds are defined in: build/windows/jhbuild/build.jhbuildrc
+  It would be a good idea to compare contents and maybe deactivate some
+  useless options from dependencies. Also if all builds end up having a
+  very similar dependency list, maybe there could be a meta-format from
+  which both the jhbuild and the flatpak manifest could be generated.