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

+[submodule "build/flatpak/flathub"]
+	path = build/flatpak/flathub
+	url = git@github.com:flathub/org.gimp.GIMP.git

flathub @ 2f9a3362

+Subproject commit 2f9a3362289727807db5b74d6ebf426d8a1f09a7

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.