[gimp] build: add the flathub stable repository as a submodule.

[Jehan Pagès <jehanp src gnome org>]

commit 580782dc659393c03f30fd93e883e745b8e5fd08
Author: Jehan <jehan girinstud io>
Date:   Wed Dec 6 15:23:19 2017 +0100

    build: add the flathub stable repository as a submodule.
    
    This will only be used for our development/nightly flatpak builds and
    this submodule does not have to be pulled for normal GIMP development.

 .gitmodules                     |    3 +++
 build/flatpak/flathub           |    1 +
 build/flatpak/flatpak-howto.txt |   10 ++++++++++
 3 files changed, 14 insertions(+), 0 deletions(-)
---
diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..0d78e97
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "build/flatpak/flathub"]
+       path = build/flatpak/flathub
+       url = git github com:flathub/org.gimp.GIMP.git
diff --git a/build/flatpak/flathub b/build/flatpak/flathub
new file mode 160000
index 0000000..2f9a336
--- /dev/null
+++ b/build/flatpak/flathub
@@ -0,0 +1 @@
+Subproject commit 2f9a3362289727807db5b74d6ebf426d8a1f09a7
diff --git a/build/flatpak/flatpak-howto.txt b/build/flatpak/flatpak-howto.txt
index 6d5ade7..36df3b5 100644
--- a/build/flatpak/flatpak-howto.txt
+++ b/build/flatpak/flatpak-howto.txt
@@ -5,6 +5,8 @@ Stable build
 ~~~~~~~~~~~~
 
 The stable manifest is maintained at flathub: https://github.com/flathub/org.gimp.GIMP
+This repository is pulled in GIMP repository as a submodule so that we
+can share as much code as possible with the other builds.
 
 Development and nightly builds
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -17,6 +19,14 @@ stable too.
 Building and maintaining
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+* Init the submodule if this is the first time:
+
+> git submodule init
+
+* Update it:
+
+> git submodule update
+
 * Install flatpak and flatpak-builder. There are packages for most
   distributions. Check: http://flatpak.org/getting.html
   Use at least flatpak 0.9.5.