[gimp-web] Add button for nightly flatpak in development download page.

[Jehan <jehanp src gnome org>]

commit b64c98e7360ff6aac8f43e787f731b1472d462cb
Author: Jehan <jehan girinstud io>
Date:   Thu May 27 00:13:09 2021 +0200

    Add button for nightly flatpak in development download page.

 .../newgimp/templates/downloads-development.html   | 24 +++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)
---
diff --git a/themes/newgimp/templates/downloads-development.html 
b/themes/newgimp/templates/downloads-development.html
index 9a352cf9..4ca28256 100644
--- a/themes/newgimp/templates/downloads-development.html
+++ b/themes/newgimp/templates/downloads-development.html
@@ -57,17 +57,27 @@
     <div id='linux' class="os linux" markdown='1'>
         <h2>GIMP (development version) for Unix-like systems</h2>
 
-        <p>We provide a Flatpak development repository on Flathub. This
-        is not a nightly, hence only development releases will be made
-        available (i.e. specific snapshot manually selected, not
-        automatic builds). If there are no development releases, the
-        latest stable release will be provided instead in this
-        repository.</p>
+        <p>We provide 2 Flatpak unstable builds: on Flathub for point
+        development release and on the GNOME nightly repository.
+        The development releases are manually selected and tested
+        snapshots of the code, hence deemed presentable to the public
+        whereas nightlies are untested by definition. Nightlies are
+        automated builds at random point in development process.
+        <br/>
+        If there are no development releases, the latest stable version
+        will be provided instead in the development repository whereas
+        the nightly repository will always contain moving
+        work-in-progress code.</p>
 
         <p>
             <span class='win-button'>
                 <a href="https://flathub.org/beta-repo/appstream/org.gimp.GIMP.flatpakref";>
-                    Install GIMP development flatpak
+                    Install GIMP development flatpak (unstable)
+                </a>
+            </span>
+            <span class='win-button'>
+                <a href="https://nightly.gnome.org/repo/appstream/org.gimp.GIMP.flatpakref";>
+                    Install GIMP nightly flatpak (very unstable)
                 </a>
             </span>
         </p>