[gimp-web/testing] Add a button for GIMP stable flatpak on the downloads page.

[Jehan Pagès <jehanp src gnome org>]

commit fa2612d4ddfe163b892b71a2b86f030c6c6deed3
Author: Jehan <jehan girinstud io>
Date:   Tue Oct 17 11:55:57 2017 +0200

    Add a button for GIMP stable flatpak on the downloads page.

 themes/newgimp/templates/downloads.html |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)
---
diff --git a/themes/newgimp/templates/downloads.html b/themes/newgimp/templates/downloads.html
index d0f241d..229eb0e 100644
--- a/themes/newgimp/templates/downloads.html
+++ b/themes/newgimp/templates/downloads.html
@@ -45,8 +45,16 @@
 
     <div id='linux' class="os linux" markdown='1'>
         <h2>GIMP for Unix-like systems</h2>
+        <p>
+            <span class='win-button'>
+                <a href="https://flathub.org/repo/appstream/org.gimp.GIMP.flatpakref";>
+                    Install GIMP flatpak
+                </a>
+            </span>
+        </p>
 
-        <p>It's very likely your Unix-like system such as a GNU/Linux distribution already comes with a GIMP 
package. It is a preferred method of installing GIMP, as the distribution maintainers take care of all the 
dependencies and bug fix updates. Please note that many distros decide to pin a specific version of GIMP to 
their releases.</p>
+       <p>The flatpak link above should open your software installer and propose you to install GIMP 
(available platforms: i386, x86-64, ARM and AArch64). It may not work out-of-the-box on some platforms since 
the flatpak technology is recent. If that is the case, <a href="http://flatpak.org/getting.html";>ensure 
flatpak is installed</a> and if just clicking the link still does not propose to install GIMP, install by 
command line: <code>flatpak install https://flathub.org/repo/appstream/org.gimp.GIMP.flatpakref</code></p>
+        <p>The flatpak build is very new and therefore may present shortcomings. It's very likely your 
Unix-like system such as a GNU/Linux distribution already comes with a GIMP package. It is a preferred method 
of installing GIMP, as the distribution maintainers take care of all the dependencies and bug fix updates. 
Nevertheless note that many distros decide to pin a specific version of GIMP to their releases, whereas our 
flatpak will follow GIMP releases closely.</p>
 
         <p>
             GIMP can also run on Solaris and is available for the BSD family of systems such as FreeBSD and 
OpenBSD.